Make gravatar link on client-side

Link to gravatar image is generated completely on client-side and no
longer included into user or record objects.

Added libs:
 * JS MD5 (BSD license)
 * JS jQuery.gravatar (WTFPL license), changed

Removed libs:
 * Flask.Gravatar

Closes bug 1320242

Change-Id: Iee08dc845c162a3ccb97d176997df31a6e7b3c63