Session stored globally per request

What

ApplicationController stores the session so it can be globally accessible per request, in a similar way to how we can access other data stores (redis/postgres) from anywhere.

• Session.current can be used to access the session, and is cleaned up after the request ends.
• NamespacedSessionStore can be used to store data under a given key.
• DynamicStore provides flexibility for setting / retrieving data outside of a web request via a Hash

Why

Used from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10034 where the alternative might have been to pass session data around with the user. This is cleaner and can be reused for other things.

Conformity

• Code review guidelines
• Merge request performance guidelines

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers