[WIP] Refactoring for Snippets

- Make Structure a super type
- "Page" will take role of current Structure
- Snippet will be subclass of Structure

## Depends

- [x] https://github.com/sulu-cmf/SuluTestBundle/pull/25 
- [ ] https://github.com/sulu-cmf/SuluTestBundle/pull/26

## Todo

- <strike> Sub-class the template reader. </strike> (handled in separate PR)
- <strike> Enforce the `title` property on Snippet templates.</strike>
- [x] Webspace should not be required by the API
- [ ] Either refactor the ContentMapper test or not. but in anycase, add Snippet tests^
- [x] Handle "not found" snippets when retrieving multiple snippets by ID (do not 500) - (fixed in SnippetBundle)

^ The snippet mapping is already well tested by the SnippetBundle, but we should add tests here too.
Also, using repository initializers, each Initializer would be in its own bundle. So the SnippetInitializer would only be executed when the SnippetBundle is active.

I think we can fix this properly by finishing the above PR later, WDYT? /cc @wachterjohannes @chirimoya