linkifier: Support URL templates for linkifiers.

This swaps out url_format_string from all of our APIs and replaces it
with url_template. Note that the documentation changes in the following
commits  will be squashed with this commit.

We change the "url_format" key to "url_template" for the
realm_linkifiers events in event_schema, along with updating
LinkifierDict. "url_template" is the name chosen to normalize
mixed usages of "url_format_string" and "url_format" throughout
the backend.

The markdown processor is updated to stop handling the format string
interpolation and delegate the task template expansion to the uri_template
library instead.

This change affects many test cases. We mostly just replace "%(name)s"
with "{name}", "url_format_string" with "url_template" to make sure that
they still pass. There are some test cases dedicated for testing "%"
escaping, which aren't relevant anymore and are subject to removal.
But for now we keep most of them as-is, and make sure that "%" is always
escaped since we do not use it for variable substitution any more.

Since url_format_string is not populated anymore, a migration is created
to remove this field entirely, and make url_template non-nullable since
we will always populate it. Note that it is possible to have
url_template being null after migration 0422 and before 0424, but
in practice, url_template will not be None after backfilling and the
backend now is always setting url_template.

With the removal of url_format_string, RealmFilter model will now be cleaned
with URL template checks, and the old checks for escapes are removed.

We also modified RealmFilter.clean to skip the validation when the
url_template is invalid. This avoids raising mulitple ValidationError's
when calling full_clean on a linkifier. But we might eventually want to
have a more centric approach to data validation instead of having
the same validation in both the clean method and the validator.

Fixes #23124.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/lib/home.py

@@ -138,6 +138,7 @@ def build_page_params_for_home_page_load(
         "user_avatar_url_field_optional": True,
         "stream_typing_notifications": False,  # Set this to True when frontend support is implemented.
         "user_settings_object": True,
+        "linkifier_url_template": True,
     }
 
     if user_profile is not None: