Follow-up: Add indicator while waiting on "change username" request

At the moment we are just disabling the user input and the "update username" button when a user changes the username:

There probably should be an spinner or another indicator.

The following discussion from gitlab-ce!17405 should be addressed:

• @winh started a discussion: (+1 comment)

  @leipert This seems to be me more of a UX thing: Other than disabling the input, there is no indication for a user that a request is pending. I would prefer displaying a spinner somewhere. Given that it was not part of the original spec, we can make that a new issue.