Remove statistics for pages and user visits

Currently Crabgrass counts various actions like page visits, user visits and group visits.

The counts are used in two places:

• Page details: Views, Stars, Edits for today, this week, this month and all time
• Group members: number of Visits

The statics were introduced long time ago. Today we do not find them helpful anymore. We also want to reduce the data stored and get rid of unnecessary complexity.