This commit implements the sort functionality for the user lists (by name, email, last_activity) and ordergroup lists (by name).
It is a first attempt addressing issue #560.
This change introduces two new data types to group the financial
transactions. Now every transaction has a "type", which itself belongs
to a "class".
Types should be used add structured information to an transaction, instead
of writing it into the notice textfield. E.g. this could be used to have
different types depending on the source of money (cash vs. bank transfer).
Classes are shown as different columns in the tables and will be uses to
group transactions of specific types. They should be used if not the whole
amount of ordergroup should be used to order food. E.g. if there is a
deposit or membership fee, which is independent of the normal credit.
This will allow us to implement additional features based on classes in
the future. E.g. the sum of transactions in the "membership fee" class
must be positive to allow food orders or show a big warning if it is bellow
a certain value.
