We perform a ton of expensive setup in this spec, via both `before` and
`let!`. When we use `skip` inside an `it` block, rather than skipping
the block entirely via `xit`, we still perform this expensive setup,
only to do absolutely nothing with it, resulting in wasted time.
Minutes, in this case.

Further, these specs have been sitting in our test suite for about seven
months now, so the likelihood of them being addressed soon seems low.

Signed-off-by: NRémy Coutable <remy@rymai.me>

Removed all the unnecessary include of `WaitForAjax` and `ApiHelpers` in the specs.
Removed unnecessary usage of `api:true`

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/27262

This is a proof of concept for gitlab-org/gitlab-ce#30196.

The actual login procedure is well-tested by
`spec/features/login_spec.rb`, and we don't gain anything by also
thoroughly testing it here, in our second-slowest feature spec. In fact,
it only slows us down!

So instead we use `sign_in` from the `Devise::Test::IntegrationHelpers`
module, which just sets the current user at the Warden level. This
drastically reduces the "setup" phase of every test in this file. A
non-scientific test run saw this drop from 633 to 231 seconds.

Signed-off-by: NRémy Coutable <remy@rymai.me>

Fixed edge cases with changing value