Better theme support

Some themes allow you to customize images and callouts with metadata after a "|" symbol. For example this in the ITS theme:

[!infobox|wikipedia]
![[image.png|banner|200]]

This release adds support for this syntax. You also need to update the template for this to work properly.
PR: #443

Delete unused images

Images that are no longer used should now be deleted
PR: #427

Better performance

Some changes have been done to make various actions faster in some curcumstances. See PRs for details.
PRs:
#433
#434
#441

Better dev experience

Changes that have enhanced the developer experience
PRs:
#435
#442