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/help_relative_links.py

@@ -5,6 +5,8 @@ from markdown import Markdown
 from markdown.extensions import Extension
 from markdown.preprocessors import Preprocessor
 
+from zerver.lib.markdown.preprocessor_priorities import PREPROCESSOR_PRIORITES
+
 # There is a lot of duplicated code between this file and
 # help_settings_links.py. So if you're making a change here consider making
 # it there as well.
@@ -73,7 +75,9 @@ class RelativeLinksHelpExtension(Extension):
     def extendMarkdown(self, md: Markdown) -> None:
         """Add RelativeLinksHelpExtension to the Markdown instance."""
         md.registerExtension(self)
-        md.preprocessors.register(RelativeLinks(), "help_relative_links", 520)
+        md.preprocessors.register(
+            RelativeLinks(), "help_relative_links", PREPROCESSOR_PRIORITES["help_relative_links"]
+        )
 
 
 relative_help_links: Optional[bool] = None