Implement and verify metrics pings about side-view

[ianb]

Some notes / questions:

1. We don't know the sidebar width at the time we start opening a page, only when the sidebar is later opened. So it's hard to fill in cd1 at the "normal" location. Answer: this is fine
2. There doesn't seem to be a reasonable way to distinguish between the tab context menu and the page context menu. Answer: no choice
3. Do we want a startup ping? Answer: yes
4. We don't have a keyboard shortcut. Answer: skip it?
5. should showing the homepage be a load-url event? Answer: OK
6. We're missing a browserAction event Answer: document what's there
7. Are we cool not bucketing sidebar widths (pixels)? Answer: bucket to nearest 50px
8. Add an event for when we can't save something to recentTabs? Answer: yes, plus other failures