Added search index admin to be able to easier handle search index changes. Fix #1930.

There are still a few things needed to complete this:

- [x] Write docs
- [x] Add screencast for you all to see the workflow
- [x] Fix and extend tests a bit
- [x] Handle update to the old index while the new is being populated and not yet promoted
- [x] Open webops bug
- [x] fix @robhudson's review finds