Bug 37853: Do not POST a form at the end of changing the password (OPAC)

Bug 37853: Do not POST a form at the end of changing the password (OPAC)

Rather than having a form that does a POST (without an op, which isn't proper),
the final step in changing your password in the OPAC should just be a link.

Test plan:
1. Without the patch applied, log in to the OPAC as a user that you don't
   mind changing the password for
2. Welcome, {username} - Your account - Change password
3. Type the old password, and a new password twice, click Change password
4. Below the success message is a green button with white text, Return to
   my account - click it
5. Apply patch
6. Change password, type the (new) old password, and a (newer) new password
   twice, click Change password
7. Below the success message is what looks like a green button with white
   text, Return to my account
8. Hover the button, check your browser status bar for a URL showing you
   that it's now a link. Click it to be sure it works.

Sponsored-by: Chetco Community Public Library
Signed-off-by: Olivier V <olivier.vezina@inLibro.com>
Amended-by: Jonathan Druart
Edited commit message

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>