ttrss: Use database export on package uninstall

As per #556, this PR changes two things: it adds a database export option, and will _always_ remove the database from MySQL during package uninstall.

This way, reinstalling the same package won't cause issues(e.g. #1246, https://github.com/SynoCommunity/spksrc/issues/556#issuecomment-56098717), and it still allows the data to be available in one form or the other.

Note this PR (for now) only updates ttrss spk. Any thoughts, before I change the rest of the packages?