Fix 595-118, low quantity filter on inventory page

I had to consolidate two tables into a single database (as it stood each table was in its own database).
Also, all quantity additions/subtractions now result in using the original unit for the final value. This is because the sophisticated conversion between units was not possible within the SQL query. needed to check for InventoryTypes with low quantity.
The checkbox needs a label and better styling. Since I do not have enough UI experience I think someone else could do this better. I will create a task for it.