Bug 33820: Add hints to warn the librarian that they will be logged out if they chang...

Bug 33820: Add hints to warn the librarian that they will be logged out if they change their username

This patch adds some JavaScript to the patron entry form which adds a
hint if the logged-in librarian tries to change their own username. It
warns them that they will be logged out.

To test, apply the patch and log into the staff interface.

- Locate and edit your own patron record.
- In the "OPAC/Staff interface login" section, change the value in the
  username field. When focus leaves that form field a hint should
  appear, "You will be logged out if you modify your username."
- Change the username field back to your original username. The hint
  should disappear.
- Confirm that submitting a username change works correctly: You should
  be logged out, and when you log in again your username should be
  updated.

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>