Refactor conversations css + fix inconsistent height of conversations list bug

This improves https://github.com/diaspora/diaspora/issues/3796 and fix a bug where the list of the conversations was not displayed because the height was forced to 0. The display is now done only with CSS.

**Not ready to merge** I still have work to do, especially, the tooltip for the timeago is not displayed correctly. The width is hardset somewhere, I don't find where, if someone knows...