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/fenced_code.py

@@ -88,6 +88,7 @@ from pygments.lexers import find_lexer_class_by_name
 from pygments.util import ClassNotFound
 
 from zerver.lib.exceptions import MarkdownRenderingException
+from zerver.lib.markdown.preprocessor_priorities import PREPROCESSOR_PRIORITES
 from zerver.lib.tex import render_tex
 
 # Global vars
@@ -162,7 +163,9 @@ class FencedCodeExtension(Extension):
         processor = FencedBlockPreprocessor(
             md, run_content_validators=self.config["run_content_validators"][0]
         )
-        md.preprocessors.register(processor, "fenced_code_block", 25)
+        md.preprocessors.register(
+            processor, "fenced_code_block", PREPROCESSOR_PRIORITES["fenced_code_block"]
+        )
 
 
 class ZulipBaseHandler: