Resolve "Application Token"

Closes #1654 (closed)

Implements endpoint /app_token?expires_in=<int> for simple tokens like the signature token to access every endpoint available with the Keycloak access token via the Authorization: Bearer <app token> header. Adds tests for requesting the both app and signature token with and without authorization. Adds a single test to access a restricted endpoint (/users/me) with the app token.