Magazine setting, so only mods can create threads and microblog posts

[BentiGorlich]

Is your feature request related to a problem? Please describe. There are a few use cases where you want to restrict the creation of threads and microblogs (not comments on them) to a handful of people

Describe the solution you'd like For such a use case there should be options in the magazine settings for:

• restrict creation of threads to mods
• and the same for microblogs

Additional context I want only the moderators (i.e. us devs of mbin) to be allowed to create threads in https://gehirneimer.de/m/mbinReleases