rework user profiles

This implements #430

Most of the work in this branch was originally done by @nidico. Their work can still be seen in the [original branch](https://github.com/nidico/adhocracy/tree/feature-430-rework-user-profiles). I took over the work to tie up all loose ends. To make review easier I rebased some of my changes into @nidoco's commits.

This pull request includes multiple weakly connected tickets:

-   new event pager design
-   new dashboard and profile page
-   move top_action buttons to better places
-   include truncated previews in event pager rows

I am not sure about all of the details, e.g. where to move the top_action buttons. So I am open for discussions.

What is missing:

-   avatars (will talk to @joka about image upload)
-   sectionpage links (and therefore events) are broken (see #605)