page index as tiles

*this is rebased against #712*

This adds an instance option to use tiles (see #687) instead of a pager on the page index. In that case it is also possible to upload a logo for a page which is then shown in the tile.

Note that currently no pagination is implemented, meaning that this will not scale to many pages. However the cases for which this was designed always had a fixed set of pages (<10).

I did not add code for deleting the logo on page deletion because I did not know if this is needed.