Task list improvements

[ublefo]

Some of the small UX changes I would like to propose, these probably don't need separate issues for now.

1. Currently it's not possible to filter for tasks that belong to a particular grade without changing the target grade. This sends a request to the server and changes it on the server side as well, it would be nice to have a temporary filtering option that only affects client side display integrated into the search bar.
2. We should consider providing alternative sorting options beside the default (sorting by start time), such as by abbreviation. Some units might have long-running tasks and these will make the list look really disorganized.
3. There should be a clear visual separation between ongoing (past start date) and future (start date is in the future) tasks.