Improves the Snippet Entity

The following features have been implemented or are missing.

## REST API
- [x] ```GET /snippets```
- [x] ```GET /snippets/{id}``` <br />
```GET /articles/{articleNumber}/{languageCode}/snippets/{snippetId}```
- [x] ```GET /snippets/{id}/rendered/{show}```
- [x] ```GET /articles/{articleNumber}/{languageCode}/snippets```
- [x] ```POST /snippets/``` <br />
```POST /articles/{articleNumber}/{languageCode}/snippets```
- [x] ```POST|PATCH /snippets/{id}``` <br />
```POST|PATCH /articles/{articleNumber}/{languageCode}/snippets/{snippetId}```
- [x] ```DELETE /snippets/{id}```
- [x] ```LINK /articles/{articleNumber}/{languageCode}```
- [x] ```UNLINK /articles/{articleNumber}/{languageCode}```
- [x] ```GET /snippetTemplates/{show}```
- [x] ```GET /snippetTemplates/{id}/{show}```
- [x] ```POST /snippetTemplates/```
- [x] ```PATCH /snippetTemplates/{id}```
- [x] ```DELETE /snippetTemplates/{id}```
- [ ] Finding a specific Snippet by name (Implemented in the Repo, not exposed)

## Snippet Entity/Repo
- [x] Implement the Controller option for the Snippet

## Newscoop frontend
- [x] Article parsing for finding Snippet embed
- [x] ~~Render Snippet Embed for the Frontend through Smarty~~

There is a bug which prevents ```PATCH``` from working. This is known, use ```POST```.