provide a query log

[kohlhase]

It would be very interesting to have an overview over what kind of queries are used. @tkw1536 says we should implement this in the TeMaSearch proxy. Here are some concerns.

• logs should be anonymous
• we need to document shat we log in the front-end (privacy policy()
• we should have a tool that does interesting things with the log (top queries, ... )

This is something for @typohnebild