We move robots.txt to custom/ instead of custom/public/ as
custom/public/ is now served at /assets/ via the gitea webserver and we
need robots.txt at the root. Related to this we update image urls to be
prefixed with AssetUrlPrefix in their paths so that if this path changes
against in the future we should automatically accomodate that.
Change-Id: I8ce5fe8ff342617ff156a401be8418d593fd35c4
048a918a61