[2.x] Slideover header button fix

[Jade-GG]

The back arrow button used id instead of for and used a weird attributes merge (that didn't include the class in the merge, which broke it). There may have been a use case for it while developing this slideover, but it sure as heck isn't used now.

[royduin]

So we don't want any attribute merging?

[Jade-GG]

So we don't want any attribute merging?

Not on that label, no. This header is used via an @include in the index, where $attributes is actually used one DOM level up for the styling of the whole slideover.