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

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

Cherry-picked d2528b26b6da34f34b5d7a392e263428d3c09d69

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.