Add tools/check-css and tools/lib/css_parser.py.

`tools/lint-all` now calls the new `tools/check-css`

The css_parser library parsers CSS into a data structure
that remembers line numbers and columns of semantically
meaningful tokens and adjoining white space/tokens.  It
is intended to be used for various linting tasks.

The file `tools/check-css` runs a few files through the
parser and makes sure they round trip.  This has some value
right away, as files that fail to parse will cause an
exception to be thrown and thus alert developers to syntax
errors.  We expect to grow this into more advanced linting
tasks eventually.

tools/check-css

@@ -0,0 +1,40 @@
+#!/usr/bin/env python
+from __future__ import absolute_import
+from __future__ import print_function
+from lib.css_parser import parse, CssParserException
+import os
+import sys
+import glob
+
+try:
+    import lister
+    from typing import cast, Callable, Dict, Iterable, List
+except ImportError as e:
+    print("ImportError: {}".format(e))
+    print("You need to run the Zulip linters inside a Zulip dev environment.")
+    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
+    sys.exit(1)
+
+def validate(fn):
+    # type: (str) -> None
+    text = open(fn).read()
+    section_list = parse(text)
+    if text != section_list.text():
+        print('BOO! %s broken' % (fn,))
+        open('foo.txt', 'w').write(section_list.text())
+        os.system('diff %s foo.txt' % (fn,))
+        sys.exit(1)
+
+def check_our_files():
+    # type: () -> None
+    fns = glob.glob('static/styles/*.css')
+    for fn in fns:
+        try:
+            validate(fn)
+        except CssParserException as e:
+            print('CssParserException raised while parsing file %s' % (fn,))
+            print(e)
+            sys.exit(1)
+
+if __name__ == '__main__':
+    check_our_files()