[3.0] Theme split (wave 8, part 4) — clear floats with inline-start and inline-end

[albertlast]

Description

Part 4 of wave 8 of the #7933 split.

clear: inline-start and clear: inline-end follow the writing direction, so the three
places where rtl.css did nothing but mirror the side no longer need an override.
clear: both is unaffected and untouched.

The one group left alone, and why

index.css clears five selectors together — .signature, .attachments,
.under_message, .custom_fields_above_signature and .custom_fields_below_signature —
but rtl.css mirrors only three of them. Converting the group would also start mirroring
.under_message and .custom_fields_below_signature, which is a behaviour change, not
a no-op, so the group keeps its physical value here.

Whether those two are missing from rtl.css by accident is worth deciding on its own.
It is not a question about dead code — all three of the classes involved are emitted by
the templates, so the asymmetry is real and visible in a right-to-left language.

How this was checked

Same method as parts 2 and 3, on 29 pages captured back to back across a stash.

One addition worth noting: a poll had to be posted and voted on first. The
#poll_options dl.options dt rule is one of the three being converted, and none of the
existing sweep pages reached it — the results list only renders once a vote has been cast,
so before that the check would have passed without ever exercising the rule. With the poll
in place the sweep covers all three.

	elements	differences
left-to-right	9735	5, all the clear keyword
right-to-left	9735	5, all the clear keyword

No geometry moved in either direction.

rtl.css goes from 453 lines to 442.

Issues References (Fixes|Related|Closes)

Related to #7933.

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On the ordering of these four

These were originally stacked. They are not any more — each of #9569, #9570, #9571 and
#9572 now branches from release-3.0 on its own and contains exactly one commit, so each
diff shows only its own property and nothing else. They can be reviewed in any order.

They cannot all be merged without a rebase in between, and that is worth being straight
about. One rtl.css rule usually mirrors several properties at once, so these PRs edit
some of the same declaration blocks. release-3.0 has:

#poll_options dl.options dt { float: right; clear: right; }
#poll_options dl.options dd { float: right; text-align: left; }

#9569 removes both floats, #9570 removes the text-align, #9571 removes the clear —
and a rule only disappears once its last declaration is gone. Merging any one of these is
fine; after that I will rebase the rest, which is a few minutes of work each time. Of the
six pairs, text-align+clear and clear+margins merge cleanly as they stand; the
float one overlaps with all three, so merging that one first costs the least.