Export requestlog and improve formats for the exporting

Added `gzip`, `tgz (tar.gz)` and `tbz (tar.bz2)` to compression methods for exporting
Added mime-types for the contents to the response.

Changed rendering in an abstraction method `_render_json`,
to implement further render methods for `protobuf` or other similar structured datatypes.

As requested by [adhocracy.hhu_theme/issues/#350](https://github.com/hhucn/adhocracy.hhu_theme/issues/350)