[Backend] Make encryption backend pluggable

* Use Factories to get (Encrypted)DbStorage instances
* Enable configuration file option to turn encryption on or off
* Removed FilesystemBackend from the default settings.conf (it can still be configured but is undocumented)
Fix wysiwyg parser rules

* Explicitly specify what tags are allowed (whitelist)
* Use <strike> instead of <del> (also done in the backend)