Rebased !243

This MR is a fork of !243 (merged), which is currently stalled, so please read it's changelog first.

Differences from !243 (merged)

• Rebased
• Added new mix tasks that didn't exist when !243 (merged) was proposed (relay, migrate_uploads) and updated existing ones (new config opts in instance config, the disappearance of info_changeset)
• Removed pleroma.gen. pleroma.gen.invite was moved into pleroma.user invite pleroma.gen.instance was moved into pleroma.instance gen
• Refactored some copy-paste code to private functions
• Added Mix.Tasks.Pleroma.Common which houses functions that can be shared between tasks (starting pleroma, getting options)