webapp: add tooltip on repo connection icon

#git-annex:
<greg-g> What does the signal strength icon next to a remote in the webapp mean?
<ion> It signifies a remote from which git-annex will get instant change notifications.
<greg-g> neat
<greg-g> I thought that was it, but wasn't sure (there's no hover text/tip)
<ion> A hover title would be good indeed.