cmd/bosun replace textarea with aceeditor in expr

[KozzyKoder]

Hello,

We have UX troubles trying to utilize Expr editor which is basically just textarea: it doesn't have syntax highlight and monospace font.
That's why we've implemented AceEditor to expr editor.

this is reimplementation of another PR
which seems to be abandoned: #2201

Thanks!

[kylebrandt]

I do like changing this to the ace editor, however I'd like to see two changes:

• The expression page supports "shift-enter" to execute the expression, would like to keep that ability
• I think the default height should be a little smaller (although probably a few more lines then current) so by default there is more screen real estate for results.

[kylebrandt]

Oh, also the bosun logo animation seems to keep going until you run an expression, I think there is stop JS function to stop that on load

[KozzyKoder]

@kylebrandt fixed review issues