Change gitlab_sign_in to sign_in

The exciting conclusion of https://gitlab.com/gitlab-org/gitlab-ce/issues/30196#note_31552752!

I changed a few of our biggest offenders by hand and ran before-and-afters, just to give an idea of the improvement we should expect:

$ be rspec spec/features/discussion_comments/
	Finished in 4 minutes 19.7 seconds (files took 11.88 seconds to load)
	Finished in 2 minutes 30.2 seconds (files took 10.52 seconds to load)

$ be rspec spec/features/projects/pipelines/pipelines_spec.rb
	Finished in 3 minutes 41.7 seconds (files took 14.56 seconds to load)
	Finished in 2 minutes 21.2 seconds (files took 13.88 seconds to load)

$ be rspec spec/features/issues/user_uses_slash_commands_spec.rb
	Finished in 6 minutes 47 seconds (files took 17.55 seconds to load)
	Finished in 4 minutes 1.4 seconds (files took 12.44 seconds to load)
		(also includes removal of duped specs in 17196a2ff31)

$ be rspec spec/features/merge_requests/user_uses_slash_commands_spec.rb
	Finished in 3 minutes 40.9 seconds (files took 18.16 seconds to load)
	Finished in 2 minutes 45 seconds (files took 11.8 seconds to load)

TODO:

• Fix specs broken by this change
• gitlab_sign_out to sign_out
• gitlab_sign_out_direct to sign_out
• Remove gitlab_sign_out_direct definition
• Fix specs again
• EE version, do it all over again 🎉 ➡ https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2255