Revert "WIP: add data-src attribute to attributes whitelist in angularJS html sanitization."

This reverts commit 962d28f193803a85327188f2cb47ad762b615325.
Mark entry contents and summaries as explicitly trusted when binding them into the AngularJS view.

This is necessary because by default the ng-bind-html directive removes the data-src attribute from images in entries; this attribute is necessary for the jquery-unveil library (for image lazy-loading) to work. Explicitly marking the html in entry summaries and contents as trusted, it is inserted in the view without modifications.
WIP: lazy loading images.

It's starting to look like the best option is to implement this in an angular service, instead of using js libraries. This would allow other functionalities to be added (adding a class with margins and display-block only if the image is big enough, try to load the image as a relative URL in the feed server if the first attempt to load it fails).

Not yet implemented:

- remove all traces of jquery-unveil library (no longer used)
- move lazy-loading code to its own angularjs service
- implement the above additional features
- implement a lazy-loading solution when user has requested all entries to be open by default