Application limits for indexed Elasticsearch fields

Since we see problems due to large index payloads and currently the size of fields like issue description, MR description and comments are all unlimited we can easily take down the Elasticsearch cluster by trying to index these and indeed that did happen in #199887 (closed)

A such we should add an application limit for all indexed fields to reduce the likelihood of this happening.

This will become increasingly important as we increase our rollout and also will help with keeping our ES index size down.