Optimize project list extra column queries / data caching

[mikkonie]

After #551 this should be the next step. If each app implementing a custom colonies queries the Django db for each project displayed in the list, this can generate a whole lot of queries and slow down loading of the page with a large number of results.

Possible approaches:

• Use sodarcache to save data when it changes, only e.g. query for all apps once per project
• Trigger queries from JQuery after the project list itself has loaded

Should not be attempted for v0.8.2, postponing for the next major release.

[mikkonie]

Since v0.10.9 this is not such a major issue, since the extra columns are loaded on client side and don't slow down entire page loading. That said, with complex queries this might still be inefficient, so I will keep this issue open for future consideration.