refactor: Rename backend files to use "linkifier".

This is a prep change to eventually completely
replace the term "filter" with "linkifier" in
the codebase.

This only renames files. Code changes will be
done in further commits.

zproject/urls.py

@@ -118,8 +118,8 @@ from zerver.views.realm_domains import (
 )
 from zerver.views.realm_emoji import delete_emoji, list_emoji, upload_emoji
 from zerver.views.realm_export import delete_realm_export, export_realm, get_realm_exports
-from zerver.views.realm_filters import create_filter, delete_filter, list_filters
 from zerver.views.realm_icon import delete_icon_backend, get_icon_backend, upload_icon
+from zerver.views.realm_linkifiers import create_filter, delete_filter, list_filters
 from zerver.views.realm_logo import delete_logo_backend, get_logo_backend, upload_logo
 from zerver.views.registration import (
     accounts_home,
@@ -265,7 +265,7 @@ v1_api_and_json_patterns = [
     rest_path("realm/icon", POST=upload_icon, DELETE=delete_icon_backend, GET=get_icon_backend),
     # realm/logo -> zerver.views.realm_logo
     rest_path("realm/logo", POST=upload_logo, DELETE=delete_logo_backend, GET=get_logo_backend),
-    # realm/filters -> zerver.views.realm_filters
+    # realm/filters -> zerver.views.realm_linkifiers
     rest_path("realm/filters", GET=list_filters, POST=create_filter),
     rest_path("realm/filters/<int:filter_id>", DELETE=delete_filter),
     # realm/profile_fields -> zerver.views.custom_profile_fields