Create programmatic way of defining filters

[KucharczykL]

Right now filters can only be constructed using the UI (the various FilterBar components). There are many places where items are listed only because there is no way to just link to the filtered list. Examples:

Problem

• stats page: all the tables (for example all unfinished purchases) - there is already Unfinished entry in the Purchases table where the number could be a link to the table that follows later
• view_game: ditto, all tables could be replaced with links to filtered list_x views
• last 7 days / today playimte in the navbar: both could be links to filtered list of sessions

Acceptance

• A programmatic way in Python to create a link to a combination of filters.
• Linkify the last 7 day's and today's playtime, respectively, to be links to the filtered session list

Notes

There probably should be an object that holds the filter combinations (already exists for UI?) and then at least a method to output a link similar to Django's reverse() or the url template tag.