Refactors picture filtering in admin backend and adds specs.

Moved the picture filter logic from controller to model.
Adds element name to class name selector of contents with custom tinymce config. Closes #400
Fixes page spec that expected that rails logger receives warn method.

We changed the way we log warnings in alchemy to rails debug log level in 94fcc4450339f1157f8b82fc65a25a0b429302ce

This fixes the spec for it.
JS escape validation error message when updating elements
Do not use fast_specs helper in test suite, because that breaks coverage.