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.

zerver/views/realm_linkifiers.py

@@ -0,0 +1,43 @@
+from django.core.exceptions import ValidationError
+from django.http import HttpRequest, HttpResponse
+from django.utils.translation import ugettext as _
+
+from zerver.decorator import require_realm_admin
+from zerver.lib.actions import do_add_realm_filter, do_remove_realm_filter
+from zerver.lib.request import REQ, has_request_variables
+from zerver.lib.response import json_error, json_success
+from zerver.models import RealmFilter, UserProfile, realm_filters_for_realm
+
+
+# Custom realm filters
+def list_filters(request: HttpRequest, user_profile: UserProfile) -> HttpResponse:
+    filters = realm_filters_for_realm(user_profile.realm_id)
+    return json_success({"filters": filters})
+
+
+@require_realm_admin
+@has_request_variables
+def create_filter(
+    request: HttpRequest,
+    user_profile: UserProfile,
+    pattern: str = REQ(),
+    url_format_string: str = REQ(),
+) -> HttpResponse:
+    try:
+        filter_id = do_add_realm_filter(
+            realm=user_profile.realm,
+            pattern=pattern,
+            url_format_string=url_format_string,
+        )
+        return json_success({"id": filter_id})
+    except ValidationError as e:
+        return json_error(e.messages[0], data={"errors": dict(e)})
+
+
+@require_realm_admin
+def delete_filter(request: HttpRequest, user_profile: UserProfile, filter_id: int) -> HttpResponse:
+    try:
+        do_remove_realm_filter(realm=user_profile.realm, id=filter_id)
+    except RealmFilter.DoesNotExist:
+        return json_error(_("Filter not found"))
+    return json_success()