Make HTML button strings shorter

The HTML buttons shown when viewing a bug currently have two words each.
On displays with limited horizontal width, buttons frequently split
across multiple lines which is very ugly.

Additionally, from a usability perspective, it is better to reduce the
number of words on the screen. By using only single words to describe
the action performed by each button, the interface is simplified and
less cluttered.

"Update Issue" has been changed to "Edit" as it is more specific.
"Update" could mean any number of things and doesn't particularly imply
that clicking the button lets you change/edit the bug fields.