Recognize setting for hiding of PM content in notifications

[neiljp]

What does this PR do?

This primarily fetches and dynamically updates a user setting which optionally hides private message content from appearing in notifications.

This requires fetching a new category of data (in older servers), as well as handling the update of that data.

This user setting is, along with the 24h/12h setting in a refactor, in the new user settings structure added recently.

Tested?

• Manually
• Existing tests (adapted, if necessary)
• New tests added (for any new behavior)
• Passed linting & tests (each commit)

Commit flow

• Migrate 24h/12h setting into new structure (refactor)
• Do initial fetch of new setting
• Support updating of new setting
• Use new setting to control display of content

Notes & Questions

Settings will eventually be initialized and updated differently as per #1108.

[zulipbot]

Hello @zulip/server-refactoring members, this pull request was labeled with the "area: refactoring" label, so you may want to check it out!