Can't configure ELEMENT_NAME_ACCEPTLIST.

### Expected Behaviour
I should be able to override the accepted list of elements allowed by data-sly-element.

### Actual Behaviour
Currently it's static for some reason.

### Reproduce Scenario (including but not limited to)
Doing this does not work `` and ``.

#### Platform and Version
All

Start by locating ELEMENT_NAME_ACCEPTLIST and the handling of data-sly-element in the HTL engine. Trace how the accepted element list is currently selected and determine how an override should be supplied. Done means the form reproductions work with and without the unsafe context, with coverage for the configurable list.

Written by the indexing model from the issue text.