Create a cast from @vocdoni announcing a new voting for users with high reputation

[elboletaire]

I think this should be opt-in, like the notifications feature:

• Show a switch/checkbox in the UI to create a cast via @vocdoni account
• Users with high reputation (TBD) and when enabling this feature should produce a cast via the @vocdoni account

What I'm not sure about: obviously the backend should check this reputation, so I think the best approach for the UX is to return some message when calling /create for those users with reputation below the required for this feature.

If we decide to directly disable it in the UX, this still would require a backend check to avoid malicious users altering the request. Also, some endpoint to get the reputation required for each feature would be required in order to avoid code repetition between the front and the back.