We need to rename the extension to EntitySchema.
#patch-for-review:
- [x] [PHP namespace](https://gerrit.wikimedia.org/r/506642)
- [x] [message prefix](https://gerrit.wikimedia.org/r/506996)
- [x] [autocomments](https://gerrit.wikimedia.org/r/506997)
- [x] [classes](https://gerrit.wikimedia.org/r/506998)
- [x] [ResourceLoader modules](https://gerrit.wikimedia.org/r/506999)
- [x] [link tracking](https://gerrit.wikimedia.org/r/507285)
- [x] [database table](https://gerrit.wikimedia.org/r/507286)
- [x] [content model](https://gerrit.wikimedia.org/r/507287)
- [ ] [HTML](https://gerrit.wikimedia.org/r/507288)
- [ ] [Config Variables](https://gerrit.wikimedia.org/r/507289)
- [ ] [Extension Itself](https://gerrit.wikimedia.org/r/507290) ~~(this maybe needs the repository to be moved in order to be green)~~