[SIP-194] Proposal for having an announcement banner

*Please make sure you are familiar with the SIP process documented*
[here](https://github.com/apache/superset/issues/5602). The SIP will be numbered by a committer upon acceptance.

## [SIP] Proposal for having an announcement banner

### Motivation

We wanted to tell users about announcements of varying severity.
For example: when GBQ goes down or something, we could have a banner letting them know or even when theres maintenance or an upgrade for expected downtime.

### Proposed Change

Theres going to be a button in settings bar that only admins should be able to see.
There is also going to be a modal that pops up that allows the admins to set whatever message they want.

### New or Changed Public Interfaces

There is going to be a new announcement model requiring a DB migration, an announcement API and some frontend modals and buttons to make this happen as noted in the screenshots.

### Migration Plan and Compatibility

Migration needs to happen to get the announcement data in there

Start by reading the SIP process in issue #5602, then review the proposed announcement model, database migration, API, frontend modals, settings button, and banner shown in the issue. The proposal does not name implementation files, tests, or concrete acceptance criteria; completion would require agreeing on and implementing the full design.

Written by the indexing model from the issue text.