[3.0] Line the breadcrumb up with the content it sits above

[albertlast]

Description

The breadcrumb does not line up with the content it sits above, on any page.

.navigate_section carried a margin-inline of its own — 10px — while
#main_content_section pads everything else by 12px. The breadcrumb is a sibling of
that container, not a child, so those two numbers are the only thing positioning it, and
they disagree. Result: the bar sits two pixels outside the boxes below it, everywhere.

The lower breadcrumb is further out still. Display.template.php and
MessageIndex.template.php draw a second one inside the main container, where it picks
up the 12px of padding and then adds its own 10px on top — leaving it ten pixels further
in than everything above it.

Both come from the same assumption: that the breadcrumb has to indent itself. Only the
outer one does, because only it sits outside the padded container. So that is the only one
that now carries a margin, and it carries the same 12px the container uses.

Measured at 1280px wide:

	before	after
breadcrumb above the content	73.88	75.88
content boxes	75.88	75.88
lower breadcrumb (board, topic)	86.40	76.40

How this was checked

Every breadcrumb on seven pages — admin, moderation, calendar, board index, topic display,
statistics and the front page — now starts and ends on exactly the same pixel as the
content boxes, in both writing directions (the property is direction-aware, so
right-to-left was checked too, and matches on both edges).

Then a sweep of 18 pages and 5855 elements, before and after, comparing the computed box
properties and the full bounding rectangle of every element:

• 333 values changed, and every one of them is a breadcrumb or something inside it
• 0 changed anywhere else — nothing reflowed off the back of it

Issues References (Fixes|Related|Closes)

Found while testing #7933; unrelated to that PR.