Save filter option choice to BE for issues and merge requests

Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/51323 to save choice to the BE.

Your selection choice is saved to your user, so that if you log into GitLab from any device, your previous selection persists. In particular:

• The first time you log into GitLab (or equivalent, when this feature is deployed for existing users), anytime you visit any issue in the instance, the default choice is Show all activity. If at any time you change that setting on any issue, that persists in the system so that next time you visit any issue, that same choice is shown.
• Exactly analogous behavior for merge requests.
• These are two separate saved choices per user per instance. So for a given user, there is one choice for all issues in the instance. And there is a separate choice for all merge requests in the instance.