- move column `tags` from `entry`, `entry_comment`, `post` and `post_comment` to its own table `hashtag` with a linking table `hashtag_link` which references the 4 tables. Migrating up and down should be possible without any hiccups via the migration
- Adjust a lot of code to use the new code to get the linked hashtags
- the hashtag table uses the `citext` type of psql. For it to work `citext` was added as a type and a mapping type to the doctrine configuration
- add hashtag banning as an admin option. Posts which already exists with the banned hashtag are not shown anymore (they are in the tag timeline) and new posts which contain a banned hashtag throw an exception when created
- add a hashtag panel, similar to the magazine panel. In the future subscriber count, subscribe button and block button can be added there as well
- remove the formrow for defining hashtags which are not part of the body. This may be reverted, but the row always confused me
- remove the hard coded related hashtags, it is an empty panel at the moment
- made some adjustments in the `SearchRepository` to filter out banned hashtags and use the new `NativeQueryAdapter` which should improve the performance of it

- put the extract method and its dependencies in a class which can be constructed in our unit tests. The `TagManager` now depends on symfony classes...

- use the internal count method instead of iterating over the array

- add the tags rework to the API methods. This was partially done before, should be complete now

- the factories for activity pub representation of entry, entry comment, post and post comment still used the `tags` property which no longer exists. They now expect to get an array of tags in the `create` method
- remove the `TagInterface` as it no longer provides any value and was only used in entries anyway. The way to get the tags of anything is the `TagLinkRepository`