[WIP] [FEATURE] Exclude templates which are not implemented in Webspace theme

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [x] refactor template manager
- [ ] add themes to webspace cached class
- [ ] adapt content form to show only included templates


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => not finished
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/202
| BC Breaks     | none
| Doc           | TODO