Lazy load expensive elements in edit mode

Problem/Motivation

Blocks, Webforms and other entities are currently provided as markup to the pagedesigner. This causes two problems:
- Generating the markup can take a long time on cache miss, delaying loading the pagedesigner.
- On editing such an element in the edit mode, the original markup is inserted, which does not match the settings on the element.

Proposed resolution

Add an endpoint to load the markup of an element for editing and add it to the editable frontend. The markup is generated on each request, including any changes done in editing.

User interface changes

- Additional REST endpoint (Pagedesigner Render)

API changes

- Additional REST endpoint (Pagedesigner Render)
- Additional additional field "remote_load" on pattern definitions.

Data model changes

- None