Stop the logout link from being clicked multiple times

[Robert Lyon]

This can be useful if the logout auth needs to interact with an external system as part of logout process so that a user doen't send multiple logout requests by clicking the button more than once

[Mahara Bot (dev-mahara)]

Patch for "master" branch: https://reviews.mahara.org/9646

[Steven (stevens-q)]

Environment tested: Master
Browser tested: Chrome

PRECONDITIONS:
------------------------
1) Mahara user exists and is logged in
2) Network speed has been lowered to a really slow speed

TEST STEPS:
------------------------
1) User click the logout link multiple times
2) Verify that link can't be clicked again during logout process ✔

Catalyst QA Approved ✔

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/9646
Committed: https://git.mahara.org/mahara/mahara/commit/34710872574209463eb024302f78781dde414a32
Submitter: Cecilia Vela Gurovic (<email address hidden>)
Branch: master

commit 34710872574209463eb024302f78781dde414a32
Author: Robert Lyon <email address hidden>
Date: Thu Mar 21 12:31:56 2019 +1300

Bug 1821104: Disable logout link on click

So that it can't be clicked again during logout process

behatnotneeded

Change-Id: I3951fa174b933e3c730e66de49fd18e559e6d88a
Signed-off-by: Robert Lyon <email address hidden>