Protected branch performance with thousands of branches

Zendesk: https://gitlab.zendesk.com/agent/tickets/66370

A reseller reported that a customer is having trouble with the protected branches page - on some projects it spins and times out. We found that on these projects there are 5,000-10,000 branches. Apparently the dropdown list is loading all branches on page load, or something similar.

We should find a way to asynchronously load the branches, or even live-filter them as a user types.

@jschatz1 Can someone on your team take a look, please?