Zoekt indexing rake task

Problem to solve

In 18.6, Zoekt is now Limited Availability. Our definition for LA is:

Have a complete user experience aligned with GitLab design standards

We currently do not have a way for users to index repositories in Zoekt via the UI or rake task.

Proposal

We should consider creating a rake task to start the indexing process like we do with elasticsearch (sudo gitlab-rake gitlab:elastic:index )