Add extra links on hover to blame lines

Issue 5 has some debate about where the blame line link should go. We
were resisting adding 2-3 extra links to every blame line, which would
be a little busy. Instead, use a CSS hover menu.

This does raise the issue of discoverability, but the target audience
is someone who is already interested in clicking on a blame line, so
hopefully their mouse is in the area already.

Moreover, hover does not play nice with non-mouse (e.g. touch) input
devices. This is a valid concern for Gitiles in general, but for the
particular application of blame, we want to make it as easy as
possible for power users on desktops to navigate the large variety of
options without cluttering the page too much.

Change-Id: I6e1e33864cef81f326490d543740ce985249b703
3 files changed