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

fix new label errors not visible in dropdown for #27287, also humanize error key, improve testing hieararchy for reuse, remove duplicate test

Added test to replicate the bug, also added a couple of helpers that can be reused
on the testing suite to resize the browser window, these helpers are located on the
spec/support/mobile_helpers.rb

When searching for the author in the assignee dropdown it now correctly returns the user

Closes #22905

Closes #19398