Integrate VRT_NU twitter account into add-on

[dagwieers]

Describe the feature

In case there are issues at VRT infrastructure and the add-on fails because of this, the user is not informed this is a general problem. We can help the user providing better error messages (#385) and redirecting them to official information.

The @VRT_NU twitter account is a good source of information of known issues. So it could be useful to allow the user to look at the most recent tweets coming from VRT NU. Optionally, it would also be nice to get notifications for VRT NU tweets.

For this it seems best to create a new service.twitter add-on that provides an API for registering and unregistering twitter accounts, but also to enable and disable tweets as notifications, and show tweets (and replies) coming from a twitter-account.

This would be equally useful to other add-ons that want such integration with official Twitter sources.

[dagwieers]

The main problem here is that it seems to be impossible to get Tweets from an account without authentication. Due to this I am not interested in implementing. Querying for tweets should be open and anonymous (possibly rate-limited) and requiring a login would defeat our purpose.