Pass categories to front page
Pass 20 apps per category to front page.

The themes should customize how many they want to show using Jinja2 template
system.
Fix get_featured (category arg removed).

This commit removes the arg category as it is not needed anymore.
Add 20 featured apps to front page.
Revert
Fix missing category
Merge pull request #701 from PyBossa/issue-682-8-apps

Issue 682 8 apps