Increase default avatar size

This is leftover from https://bugzilla.gnome.org/show_bug.cgi?id=792243 and part of Initiatives#6 (closed). Users panel scales down avatars in some cases to 96x96, but desired size is 512x512. Also it refuses to show avatar bigger than 64 kB. This MR increases avatar size to 512x512 and also remove the obsolete file limits. See the bugzilla and commit messages for more info.