This commit replaces how the diff system for products works under the
hood. The final saved diffs are the exact same as before, and the GUI
has simple received a "quick mode" checkbox.

The major update is that the temp diff table stores a lot less data,
and is accessable as a view, allowing for much easier subquerying in it.

This was also the (I think) last place the database didn't operate on
it's own, but was rather dependent on the GUI. Which makes this really
nice.

==== INSTALLATION ====

Apply the changes in diff.sql to the already existing database, which
creates all new tables, and sets the required data. Note that it doesn't
clean up the old tables. I will probably fix that in the future.