refactor: Rename zerver.lib.bugdown to zerver.lib.markdown .

This commit is first of few commita which aim to change all the
bugdown references to markdown. This commits rename the files,
file path mentions and change the imports.
Variables and other references to bugdown will be renamed in susequent
commits.

zerver/lib/markdown/include.py

@@ -0,0 +1,67 @@
+import os
+import re
+from typing import Any, Dict, List
+
+import markdown
+from markdown_include.include import IncludePreprocessor, MarkdownInclude
+
+from zerver.lib.exceptions import InvalidMarkdownIncludeStatement
+
+INC_SYNTAX = re.compile(r'\{!\s*(.+?)\s*!\}')
+
+
+class MarkdownIncludeCustom(MarkdownInclude):
+    def extendMarkdown(self, md: markdown.Markdown, md_globals: Dict[str, Any]) -> None:
+        md.preprocessors.add(
+            'include_wrapper',
+            IncludeCustomPreprocessor(md, self.getConfigs()),
+            '_begin',
+        )
+
+class IncludeCustomPreprocessor(IncludePreprocessor):
+    """
+    This is a custom implementation of the markdown_include
+    extension that checks for include statements and if the included
+    macro file does not exist or can't be opened, raises a custom
+    JsonableError exception. The rest of the functionality is identical
+    to the original markdown_include extension.
+    """
+
+    def run(self, lines: List[str]) -> List[str]:
+        done = False
+        while not done:
+            for line in lines:
+                loc = lines.index(line)
+                m = INC_SYNTAX.search(line)
+
+                if m:
+                    filename = m.group(1)
+                    filename = os.path.expanduser(filename)
+                    if not os.path.isabs(filename):
+                        filename = os.path.normpath(
+                            os.path.join(self.base_path, filename),
+                        )
+                    try:
+                        with open(filename, encoding=self.encoding) as r:
+                            text = r.readlines()
+                    except Exception as e:
+                        print(f'Warning: could not find file {filename}. Error: {e}')
+                        lines[loc] = INC_SYNTAX.sub('', line)
+                        raise InvalidMarkdownIncludeStatement(m.group(0).strip())
+
+                    line_split = INC_SYNTAX.split(line)
+                    if len(text) == 0:
+                        text.append('')
+                    for i in range(len(text)):
+                        text[i] = text[i].rstrip('\r\n')
+                    text[0] = line_split[0] + text[0]
+                    text[-1] = text[-1] + line_split[2]
+                    lines = lines[:loc] + text + lines[loc+1:]
+                    break
+            else:
+                done = True
+
+        return lines
+
+def makeExtension(*args: Any, **kwargs: str) -> MarkdownIncludeCustom:
+    return MarkdownIncludeCustom(kwargs)