Fix admin user deletion on profile

[HebaruSan]

Problem

As of #316, the user profile page now has a "Delete user" button for admins, which opens a confirmation dialog:

However, no matter what you do, the "Delete account" button in the confirmation popup doesn't work. The call to the server never happens.

Cause

Since the profile editing page has a rich text box, and the profile.coffee/profile.js client file is shared between the profile editing page and the profile viewing page, the rich text editor tries to initialize on page load and throws an exception because there are no textareas, just like in 1e130af.

This exception then stops the client code initialization, so the hooks that make the popup work are not set up.

Changes

• Now we only try to load the rich text editor if we have a textarea for it to use. This will allow the initialization to complete and the "Delete account" button will work.
• The confirmation popup now dynamically enables and disables the "Delete account" button as the user types, to provide a cue as to when the input is valid, just like on the admin user list
• The /api/user/<username>/delete now returns 200 on success instead of 400