Ensure full-width items within post content go full-width

[kjellr]

Flagging this issue from #4 (comment) to follow up later:

Due to some left/right padding rules, alignfull items within posts and pages do not actually extend to the full width of the page:

We should (hopefully) find a way to allow for full-width items to go all the way to the screen edges, while still retaining a similar measure of left/right padding for the header and other non-full-width items.

[pattonwebz]

The most common technique I have seen used to do this is with CSS similar to this:

.alignfull {
	margin: 2rem calc(50% - 50vw);
	max-width: 100vw;
	padding: 1rem 2rem;
	width: 100vw;
}

That has some implications on chrome where it can cause a horizontal scroll bar due to chrome (possibly all webkit browsers) not accounting for the vertical scrollbar when it calculates what 100vw is. I think it's worth exploring if this will be suitable and if the kinds with chrome can be worked out.

[kjellr]

Closing to consolidate this issue into #234.