Simplify using images in component templates

Currently using images in component templates is cumbersome, like:

```
"plugins": {
"da": {
"name": "carousel",
"unsafeHTML": "

"
}
}
````

We should support a simple placeholder `` which is replaced by UE with the full picture tag. The configuration would look like:

```
"plugins": {
"da": {
"name": "carousel",
"unsafeHTML": "

"
}
}
````
The backend can detect the `` tag without any children or extra attributes and expand it with a default template. This way we can stay backward compatible easily.

Start by locating the backend handling for component templates and unsafeHTML, then trace how picture tags are currently processed. Confirm that a childless picture placeholder expands to the default picture markup while the existing full markup remains supported.

Written by the indexing model from the issue text.