Fix context for SessionsController
What does this MR do?
SessionsController inherits from DeviseController, which inherits from ApplicationController by default, so adding context in that controller didn't help.
The problem here was that the chain was being interrupted by an earlier before_action, so we need to set the context in a prepend_around_action.
Fixes !26904 (merged). See gitlab-com/gl-infra/scalability#158 (comment 306431350) for more details.