Enable deletion of users via the API (BB-3728)

At the moment [[https://bitbucket.org/davidchambers/bitbucket.js|bitbucket.js]] creates a new user each time its test suite is run, but there's no way to delete these users via the API.

Once this functionality has been added to Bitbucket's API and bitbucket.js has been updated to clean up after itself, we could consider deleting the dummy accounts. The usernames are in the form {{{^*[a-z]

*$}}}, making false positives unlikely.