Add Redis check and "clear syncs" button on self hosted instances

[zachgoll]

Self hosted apps require Redis to work properly (see #2004)

A great contribution would be the addition of:

1. A "check" that runs before each request (only on self hosted instances) to verify if the app can connect to Redis properly. If not, show a warning in the UI to alert users to fix this.
2. A button that says "Clear syncs cache" on the "Self Hosted" settings page that simply updates all current syncs to failed (to get rid of the loading state)