Geo: Loading a merge request may cause an Error 500 in MergeabilityCheckService#update_merge_status

I noticed that when I viewed a merge request and looked at the Redis log, we were setting the Sidekiq status. Apparently if the merge request is in an unchecked state, this will update the merge status via MergeabilityCheckService#update_merge_status and launch an Elasticsearch update:

For Geo nodes, we can't do this, and loading the page will likely show an Error 500. We need need to check if the DB is in a read-only state.

/cc: @oswaldo