Implement EZP-21315: Implement embed tag for RichText

This PR resolves issues https://jira.ez.no/browse/EZP-21315 and https://jira.ez.no/browse/EZP-21387

Dedicated embed tags `ezembed` and `ezembedinline` are implemented.
In internal format those support configuration as an arbitrary hash structure. This is also available in XHTML edit format, anticipating for feature like [CKEditor widgets](http://docs.ckeditor.com/#!/guide/dev_widgets). Example:

```xml
<ezembed xlink:href="ezlocation://601" view="embed-inline">
  <ezconfig>
    <ezvalue key="size">medium</ezvalue>
    <ezvalue key="offset">10</ezvalue>
    <ezvalue key="limit">5</ezvalue>
  </ezconfig>
</ezembed>
```

RichText now defines `EmbedRendererInterface`, implemented in Symfony MVC layer using `FragmentHandler` with ESI strategy. This will fallback to inlining in case when reverse proxy is not available.

`EmbedRenderer` uses new actions implemented in `ViewController`: `embedContent()` and `embedLocation()`, taking care of checking permissions and handling visibility. This means existing view provider and custom controller configuration applies for them, also rendering fallback to Legacy Stack is possible.

Possible followup on this part is https://jira.ez.no/browse/EZP-22068.