"different account" notice in login form depend on request context processor

[Claude Paroz]

When the notice "You are authenticated as {{ username }}, but ..." (added as of #25163) is displayed on the admin login page, the username variable presence depends on the request context processor being present in the OPTIONS['context_processors'] key of the TEMPLATES setting. If not, you get:
You are authenticated as , but are not authorized to access this page. Would you like to login to a different account?

Maybe it's possible to feed the context with the username variable in the view instead?

[Tim Graham]

Sure, would be fine to backport that too if you'd like.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 1206eea:

Fixed #26558 -- Removed need for request context processor on admin login page.

[Tim Graham <timograham@…>]

In 90ea14a:

[1.9.x] Fixed #26558 -- Removed need for request context processor on admin login page.

Backport of 1206eea11e506c4e740ba2f0c1feaa01452d804b from master