Add context to SessionsController

Sean McGivernrequested to merge
add-context-to-sessions-controller into master

What does this MR do?

This controller doesn't inherit from ApplicationController and so doesn't use its set_current_context method. That means we're missing a context for Sidekiq jobs scheduled from this controller (emails and potentially web hooks).

For gitlab-com/gl-infra/scalability#158.

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