bugdown: Restructure Bugdown to extend Markdown from being an extension.

Since we are building our parser from scratch now:

1. We have control over which proccessor goes at what priority number.
   Thus, we have also shifted the deprecated `.add()` calls to use the
   new `.register()` calls with explicit priorities, but maintaining
   the original order that the old method generated.

2. We do not have to remove the processors added by py-markdown that
   we do not use in Zulip; we explicitly add only the processors we
   do require.

3. We can cluster the building of each type of parser in one place,
   and in the order they need to be so that when we register them,
   there is no need to sort the list. This also makes for a huge
   improvement in the readability of the code, as all the components
   of each type are registered in the same function.

These are significant performance improvements, because we save on
calls to `str.startswith` in `.add()`, all the resources taken to
generate the default to-be-removed processors and the time taken to
sort the list of processors.

Following are the profiling results for the changes made. Here, we
build 10 engines one after the other and note the time taken to build
each of them. 1st pass represents the state after this commit and 2nd
pass represent the state after some regex modifications in the commits
that follow by Steve Howell. All times are in microseconds.

| nth Engine | Old Time | 1st Pass | 2nd Pass |
| ---------- | -------- | -------- | -------- |
|          1 |  92117.0 |  81775.0 |  76710.0 |
|          2 |   1254.0 |    558.0 |    341.0 |
|          3 |   1170.0 |    472.0 |    305.0 |
|          4 |   1155.0 |    519.0 |    301.0 |
|          5 |   1170.0 |    546.0 |    326.0 |
|          6 |   1271.0 |    609.0 |    416.0 |
|          7 |   1125.0 |    459.0 |    299.0 |
|          8 |   1146.0 |    476.0 |    390.0 |
|          9 |   1274.0 |    446.0 |    301.0 |
|         10 |   1135.0 |    451.0 |    297.0 |

zerver/lib/bugdown/fenced_code.py

@@ -113,16 +113,7 @@ class FencedCodeExtension(markdown.Extension):
     def extendMarkdown(self, md: markdown.Markdown, md_globals: Dict[str, Any]) -> None:
         """ Add FencedBlockPreprocessor to the Markdown instance. """
         md.registerExtension(self)
-
-        # Newer versions of Python-Markdown (starting at 2.3?) have
-        # a normalize_whitespace preprocessor that needs to go first.
-        position = ('>normalize_whitespace'
-                    if 'normalize_whitespace' in md.preprocessors
-                    else '_begin')
-
-        md.preprocessors.add('fenced_code_block',
-                             FencedBlockPreprocessor(md),
-                             position)
+        md.preprocessors.register(FencedBlockPreprocessor(md), 'fenced_code_block', 25)
 
 
 class BaseHandler: