After removing the checkbox in #3913 the only remaining element that
looked like it was the new Show Annotations checkbox on the Alerts page.
Which in turn didn't look like the Enable query history checkout on the
graph page. So:
1. This takes the Enable query history button as canonical.
2. Updates the show annotations button code to match it.
3. Simplifies the JS for the checkbox.
This is a very minor UX change. The current "No Alert rules" present
table row has the `alert_header` class attached. This changes the cursor
and some other stuff and makes sense with the populated table but less
sense with the unpopulated table. So removing it the latter case.
There's no corresponding table column for this table header. The
placeholder link for silences was removed in e8800730.
Accordingly, regenerate `web/ui/bindata.go` by running:
make assets format
