markdown: Organize preprocessor priorities in one place.

All of our custom Markdown extensions have priorities that govern
the order in which the preprocessors will be run. It is more
convenient to have these all in one file so that you can easily
discern the order at first glance.

Thanks to Alya Abbott for reporting the bug that led to this
refactoring!

zerver/lib/markdown/include.py

@@ -6,6 +6,7 @@ import markdown
 from markdown_include.include import IncludePreprocessor, MarkdownInclude
 
 from zerver.lib.exceptions import InvalidMarkdownIncludeStatement
+from zerver.lib.markdown.preprocessor_priorities import PREPROCESSOR_PRIORITES
 
 INC_SYNTAX = re.compile(r"\{!\s*(.+?)\s*!\}")
 
@@ -15,7 +16,7 @@ class MarkdownIncludeCustom(MarkdownInclude):
         md.preprocessors.register(
             IncludeCustomPreprocessor(md, self.getConfigs()),
             "include_wrapper",
-            500,
+            PREPROCESSOR_PRIORITES["include"],
         )