Add additional safeguards when deleting user

Currently, when an administrator looks to delete a user within the internal Bitbucket user directory, although there is a window that shows up to confirm the deletion - it is not always clear what the impact is of this deletion and that this is an irreversible action.

It is requested that some additional safeguards be implemented in this window. This includes possibly adding a more detailed description of the impacts of this deletion, and potentially an added confirmation step similar to Repositories where the username of the user needs to be entered in order to confirm its deletion.