Validate content length on broadcast API

The maximum content count of a broadcast varies depending on its
encoding, so we can’t simply validate it against a schema. This commit
moves to using the validation from `notifications-utils`, and raising a
custom error response.

app/v2/broadcast/broadcast_schemas.py

@@ -40,7 +40,6 @@ post_broadcast_schema = {
         "content": {
             "type": "string",
             "minLength": 1,
-            "maxLength": 1395,
         },
         "web": {
             "type": "string",