Create a plugin to mute certain categories for new users by default

[tanius]

This is necessary to avoid notification spamming from the website content we host inside Discourse (explanations). The plugin should allow to configure which categories to mute for any new users, and the effect should be that these categories and separately also all their subcategories (if they have them) are added to the user preference for muted categories of any new user, upon account creation.

This can be added to the current "edgeryders" plugin with our extensions to the Discourse core functionality, or to a new plugin "webkit", which would be a cleaner solution.

As a workaround until this plugin exists, admin users can do the following steps every 1-2 months. Different from the plugin, this will adjust the list of muted categories for all users, though.

1. Go to setting “default categories muted” list and add the to-be-muted categories. When saving, choose to apply this historically to all users.

2. Reset the setting “default categories muted” and save again. This time, choose to only apply this for the future, not historically. This will keep the muted categories in the preferences of all individual users, but not mute them for the anonymous user, which is important for the functioning of the webkit software we're dealing with here.