Ability to set the default notifications settings

[Unoqwy]

Email notifications are currently opt-out only, which can lead to users getting unnecessary emails when web notifications can be enough for certain use cases. It would be great to have an option -as an admin- to control the default notifications settings.

By the way, the "Feature request" template tells to go on https://feedback.fider.io/, but the contributing guidelines ask to create an issue before working on a PR. Posting an idea to get approval to submit a PR seems ambiguous, hopefully this is the right way to go about it.