Make LinkManager subclass of InsertManager
Add ImageManager module

Subclass of InsertManager
Move relevant LinkManager tests to InsertManager

These are now part of Image
Remove default itemValues

As these were specific to LinkManager
Use this.context instead of data-attr
Listen to cmseditor events on LinkManager
Populate itemValues with page types
Setup LinkManager fixture with new generic insertmanager attrs
Update links views to use insertmanager data attrs