The ability to set update intervals/schedule for each filter list

### Problem Description
Currently, there is a global setting that controls the update interval for all filter lists. But, not all filter lists are created equal: some filter lists are updated multiple times a day and other filter lists are updated one or twice a week. This becomes an issue when you are dealing with multiple large filter lists (think 10s of MBs each) that are not updated frequently and should have a longer update interval time (like 7 days). Pulling these lists, unnecessarily, more frequently (following the global setting), leads to a huge amount of unnecessary network traffic that is counted against Internet usage caps (think of US ISPs that meter your home Internet traffic).

### Proposed Solution
One can set the global setting for filter lists update interval; however, there should be an option to set the update interval per filter list that would override this global setting. That way, one can set a custom update interval per list as needed, depending on how large the list is, how frequently the source is updated, network traffic considerations, etc. Or simply remove the global filter update interval option and have it set per list only, with the default being 24 hours for example, and users can customize that as needed per list.

### Alternatives Considered
Set long update interval (globally) so that all lists are only updated once a week to reduce network traffic generated by few large lists. However, this reduces the robustness of the filtering capability because more active filter lists add and modify filter rules more frequently, but you have to wait a much longer period of time to get the benefit of these updates.

### Additional Information
None.