Add server banner to web app, add GET /api/v2/instance to REST API (#19294)

Cherry-picked d2528b26b6

Conflicts:
- `app/serializers/initial_state_serializer.rb`:
  Upstream changed stuff, we had extra attributes.
  Applied upstream changes while keeping our extra attributes.
- `app/serializers/rest/instance_serializer.rb`:
  Upstream actually moved that to `app/serializers/rest/v1/instance_serializer.rb`,
  so updated that file by keeping our extra attributes, and took upstream's
  version of `app/serializers/rest/instance_serializer.rb`.
- `spec/views/about/show.html.haml_spec.rb`:
  Took upstream's version.

app/javascript/mastodon/initial_state.js

@@ -29,6 +29,5 @@ export const title = getMeta('title');
 export const cropImages = getMeta('crop_images');
 export const disableSwiping = getMeta('disable_swiping');
 export const languages = initialState && initialState.languages;
-export const server = initialState && initialState.server;
 
 export default initialState;