Make usernames unique over their lowercase values, and put validation in everywhere so two users can't do this again.

Usernames _are_ meant to be case insensitive in the system. But at no point where users could be created (except for XMLRPC users), was this actually being enforced. So eventually someone actually did this, which caused login for both users to break.

Now, all entry points for new users are checked to make sure users can't claim names whose lowercase value is the same as another user. And on postgres, we now have a unique index over LOWER(username). This isn't possible in MySQL, so MySQL users miss out (yet again).

* Force password change: if checked, users are forced to change their password on next log in. This was mandatory before, now people uploading CSV files can choose.
* E-mail users about accounts: if checked, users are sent an e-mail about their new account. This was never an option before and never happened, now it is on by default.

We send a slightly different e-mail based on whether the user needs to change their password or not. This affects the Add User script a tiny bit.

Added a line of descriptive text to the page to help admins understand what will happen on this page.

Corrected a language string when the password for the new user is in invalid form.

The name is largely irrelevant as it's only the creator of the file.

Use the new 'isFull' method of the institution class instead of manually working out if an institution is full.