linkifiers: Update API to send data using dictionaries.

* This introduces a new event type `realm_linkifiers` and
a new key for the initial data fetch of the same name.
Newer clients will be expected to use these.

* Backwards compatibility is ensured by changing neither
the current event nor the /register key. The data which
these hold is the same as before, but internally, it is
generated by processing the `realm_linkifiers` data.
We send both the old and the new event types to clients
whenever the linkifiers are changed.
Older clients will simply ignore the new event type, and
vice versa.

* The `realm/filters:GET` endpoint (which returns tuples)
is currently used by none of the official Zulip clients.
This commit replaces it with `realm/linkifiers:GET` which
returns data in the new dictionary format.
TODO: Update the `get_realm_filters` method in the API
bindings, to hit this new URL instead of the old one.

* This also updates the webapp frontend to use the newer
events and keys.

zerver/lib/event_schema.py

@@ -781,9 +781,26 @@ def check_realm_export(
     assert has_failed_timestamp == (export["failed_timestamp"] is not None)
 
 
-# This type, like other instances of TupleType, is a legacy feature of
-# a very old Zulip API; we plan to replace it with an object as those
-# are more extensible.
+realm_linkifier_type = DictType(
+    required_keys=[
+        ("pattern", str),
+        ("url_format", str),
+        ("id", int),
+    ]
+)
+
+realm_linkifiers_event = event_dict_type(
+    [
+        ("type", Equals("realm_linkifiers")),
+        ("realm_linkifiers", ListType(realm_linkifier_type)),
+    ]
+)
+check_realm_linkifiers = make_checker(realm_linkifiers_event)
+
+
+# This is a legacy event type to ensure backwards compatibility
+# for old clients. Newer clients should handle only the
+# "realm_linkifiers" event above.
 realm_filter_type = TupleType(
     [
         # we should make this an object