Fix/attachment edit list view

[menno-ll]

Issue link

Fixes #64

Description

The plugin was not correctly loaded in the attachment list view.
This was because the javascript was loaded after the imageCropPositionerEditFieldsReady event was triggered, not catching the event.
To solve it, on initialize of the javascript, check if the root element already exists, and if so initialize it directly.

How Has This Been Tested?

Visiting both the list view and grid view, and the detail pages of attachments.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.