Move wait_for_gitlab_to_respond to QA::Page::Base

What does this MR do and why?

Moves wait_for_gitlab_to_respond to QA::Page::Base

After the last change the method couldn't be used as a class method, which is expected of all methods where module_function is used (see !125262 (comment 1458143295)).

This moves the method to QA::Page::Base where it will still be available to all page objects. At the moment it's only used in QA::Page::Main::Login.

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