Refactorings after shutdown of legacy serlo.org

[kulla]

• ☂️ Umbrella Issue: Rework alias system api.serlo.org#133
• Migrate all revisions to edtr-io so that we can remove code for the markdown editor
• Create dump of current version
• Drop related content from DB
• Delete MySQL table license
• Delete table ad (horizon) and blog
• Delete taxonomyTypes besides topic and topicFolder
• delete table comment_vote
• remove all comments/threads on pages

[CodingDive]

Cross linking the issue on db migrations #4

[elbotho]

is horizon still in there?

[kulla]

is horizon still in there?

Yep, it's ad