[manual] Rename Django app from zephyr to zerver.

This needs to be deployed to both staging and prod at the same
off-peak time (and the schema migration run).

At the time it is deployed, we need to make a few changes directly in
the database:

(1) UPDATE django_content_type set app_label='zerver' where app_label='zephyr';
(2) UPDATE south_migrationhistory set app_name='zerver' where app_name='zephyr';

(imported from commit eb3fd719571740189514ef0b884738cb30df1320)

zerver/lib/html_diff.py

@@ -0,0 +1,114 @@
+from diff_match_patch import diff_match_patch
+import platform
+import logging
+
+# TODO: handle changes in link hrefs
+
+def highlight_with_class(klass, text):
+    return '<span class="%s">%s</span>' % (klass, text)
+
+def highlight_inserted(text):
+    return highlight_with_class('highlight_text_inserted', text)
+
+def highlight_deleted(text):
+    return highlight_with_class('highlight_text_deleted', text)
+
+def highlight_replaced(text):
+    return highlight_with_class('highlight_text_replaced', text)
+
+def chunkize(text, in_tag):
+    start = 0
+    idx = 0
+    chunks = []
+    for c in text:
+        if c == '<':
+            in_tag = True
+            if start != idx:
+                chunks.append(('text', text[start:idx]))
+            start = idx
+        elif c == '>':
+            in_tag = False
+            if start != idx + 1:
+                chunks.append(('tag', text[start:idx + 1]))
+            start = idx + 1
+        idx += 1
+
+    if start != idx:
+        chunks.append(('tag' if in_tag else 'text', text[start:idx]))
+    return chunks, in_tag
+
+def highlight_chunks(chunks, highlight_func):
+    retval = ''
+    for type, text in chunks:
+        if type == 'text':
+            retval += highlight_func(text)
+        else:
+            retval += text
+    return retval
+
+def verify_html(html):
+    # TODO: Actually parse the resulting HTML to ensure we don't
+    # create mal-formed markup.  This is unfortunately hard because
+    # we both want pretty strict parsing and we want to parse html5
+    # fragments.  For now, we do a basic sanity check.
+    in_tag = False
+    for c in html:
+        if c == '<':
+            if in_tag:
+                return False
+            in_tag = True
+        elif c == '>':
+            if not in_tag:
+                return False
+            in_tag = False
+    if in_tag:
+        return False
+    return True
+
+def highlight_html_differences(s1, s2):
+    differ = diff_match_patch()
+    ops = differ.diff_main(s1, s2)
+    differ.diff_cleanupSemantic(ops)
+    retval = ''
+    in_tag = False
+
+    idx = 0
+    while idx < len(ops):
+        op, text = ops[idx]
+        next_op = None
+        if idx != len(ops) - 1:
+            next_op, next_text = ops[idx + 1]
+        if op == diff_match_patch.DIFF_DELETE and next_op == diff_match_patch.DIFF_INSERT:
+            # Replace operation
+            chunks, in_tag = chunkize(next_text, in_tag)
+            retval += highlight_chunks(chunks, highlight_replaced)
+            idx += 1
+        elif op == diff_match_patch.DIFF_INSERT and next_op == diff_match_patch.DIFF_DELETE:
+            # Replace operation
+            # I have no idea whether diff_match_patch generates inserts followed
+            # by deletes, but it doesn't hurt to handle them
+            chunks, in_tag = chunkize(text, in_tag)
+            retval += highlight_chunks(chunks, highlight_replaced)
+            idx += 1
+        elif op == diff_match_patch.DIFF_DELETE:
+            retval += highlight_deleted('&nbsp;')
+        elif op == diff_match_patch.DIFF_INSERT:
+            chunks, in_tag = chunkize(text, in_tag)
+            retval += highlight_chunks(chunks, highlight_inserted)
+        elif op == diff_match_patch.DIFF_EQUAL:
+            chunks, in_tag = chunkize(text, in_tag)
+            retval += text
+        idx += 1
+
+    if not verify_html(retval):
+        from zerver.lib.actions import internal_send_message
+        # We probably want more information here
+        logging.getLogger('').error('HTML diff produced mal-formed HTML')
+
+        subject = "HTML diff failure on %s" % (platform.node(),)
+        internal_send_message("error-bot@zulip.com", "stream",
+                              "errors", subject, "HTML diff produced malformed HTML")
+        return s2
+
+    return retval
+