Instance wide moderation settings

[JohnXLivingston]

Currently, users can set various settings on their channel configuration page:

https://livingston.frama.io/peertube-plugin-livechat/documentation/user/streamers/channel/index.html

Some settings should also be forced on the instance level.
Peertube instance's admins should be able to:

• force the use of the moderation bot
• overload some bot settings (forbidden words, ...)
• force some settings (slow mode, ...)
• add some custom emojis that will be available on all channels
• ...

So, we should create a configuration page, very similar to the channel configuration page,
where admins could force or overload some parameters.

(The exact list of parameters that could be overloaded or forced still to be defined on case-by-case basis)