Notices: switch to Gridicons

[hoverduck]

I've already swapped the dismiss 'X' and the arrow-link arrow for gridicons. In order to swap out the type (success, error, warning, info (and any others we want to add like update)) icon we need to add a switch or case statement to notices/notice.jsx that spits out the correct type into the Gridicon component. Right now we just use a before element styled off of a .is-type wrapping class, which will no longer do.

Currently w/ nasty noticons:

[rickybanister]

@mtias tackled this in #1368