Port CE OpenAnnotations to Callimachus Open Source

[prototypo]

Port the OpenAnnotation implementation to Callimachus Open Source. This should include:

• Use OA Comments in the Discussion tab
• Use OA Categories and Tags to annotate any resource with a URL

Tweak some of the OA implementation's features. Specifically:

• Allow OA categories to be edited in user space by an admin. A big problem with the current location of categories is that they are overwritten upon Callimachus upgrade. We should maybe fallback to our list of default categories if the user space folder is empty.

Lastly, expose OA annotations on more Callimachus resources, such as:

• Create a global search page to find all resources with comments, all resources with particular tags, all resources with particular categories or some combination thereof. Base this on the CE "Browse Profiles" user profile search.
• Highlight resources with OA annotations in the folder view. Maybe colorize the lines or add an icon. Show a summary of the annotations on hover, something like this: