Fixes issues thrown in custom HMTL head & page content filtering when
the content is comprised of only a single HTML comment.
Adds tests to cover.
For #2804
- Added test cases to cover.
- Altered parsing logic to be a little less reliant on regex.
- Added new iamge repo method for creating from data.
- Added extension validation and additional type support.
- Done some cleanup of common operations within PageContent.
- Added message to API docs/method to mention image usage.
For #2700 and #2631.
Tools seems to fit better since the classes were a bit of a mixed bunch
and did not always manage.
Also simplified the structure of the SlugGenerator class.
Also focused EntityContext on shelves and simplified to use session
helper.