Always show 'Working ...' message

Currently 'Working ...' message is sticked to Gerrit page top border.
When user scroll down page and perform action he is not informed that
his action is currently performed and he should wait for its completion.
This could lead to multiple redundant RPC calls.

This change make 'Working ...' message relatively positioned to
browser top border, so that it is always visible for users.

Change-Id: Ic252d2e800a7d6d6828ca45fe2ea7adf50726bc8
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>