Consider adding support for system notices

[eloquence]

It would be good to consider mechanisms by which we can display system notices to the client, for example:

• A database integrity issue was detected. Please contact your admin.
• Your SecureDrop is running v3 onion services. An upgrade to v3000 is required.

Issues to consider:

• There may be more than one such notice at a time.
• Notices will need to eventually disappear when some condition is met.

A simple mechanism might be a feed that is downloaded as part of a sync -- if the notification is present in the feed, it will continue to be shown; if it is not, it will be deleted.

Any such mechanism is likely to require changes to the API.

[zenmonkeykstop]

Also relevant to potential system integrity checks.

There's some outstanding scope questions:

• do we want to push notices from server to client?
• do we want to push notices from FPF to clients?
• do we use actual system notifications or roll-your-own in the client or updater?

[eloquence]

Milestoning to alpha for rewrite, but needs a fair bit of scoping -- we'll need to decide one way or another what kind of messages we expect to display in sd-app and how to manage them.