Provide a way for admins to reindex all indexes with Elasticsearch from interface

For users on GitHost and other instances, running the Rake task to reindex all Elasticsearch indexes is not feasible. Could we provide a way to launch this task from the admin console?

ZD: https://gitlab.zendesk.com/agent/tickets/80625

/cc: @nick.thomas, @vsizov, @lbot, @victorwu