[3.0] Theme split (wave 8, part 3) — align text with start and end instead of left and right

[albertlast]

Description

Part 3 of wave 8 of the #7933 split.

text-align: start and text-align: end follow the writing direction, so the eleven
places where rtl.css did nothing except mirror the alignment no longer need an override
at all.

Same rule as part 2: only exact mirrors are converted — the base stylesheet says
left and rtl.css says right, or the reverse.

Left alone on purpose:

• alignments rtl.css sets where the base sheet has none, which a single logical
  declaration cannot express
• the deliberate ones. code.bbc_code and pre.file_content stay text-align: left
  in a right-to-left language because code should not be mirrored. That rule is untouched,
  and converting it would have been a real bug rather than a no-op.

Two that needed care rather than a substitution

#main_grid td.days shares a rule with #month_grid td.days, and only the first has a
mirror. It turned out to be safe anyway: the mini calendar re-declares
text-align: center immediately below at equal specificity, so the shared value never
reaches it.

rtl.css aligned #stats tr.windowbg th.lefttext and #stats tr.titlebg th.lefttext
in one rule, and only the windowbg half has a rule to mirror — the titlebg half is
aligned by the generic .lefttext instead. Removing the pair wholesale would have quietly
dropped the flip for titlebg, so that half keeps its override, now standing alone with a
comment explaining why.

How this was checked

Same criterion and method as part 2 — nothing moves in either direction — with
text-align and direction recorded alongside the geometry, on 28 pages, captured back
to back across a stash.

	elements	differences
left-to-right	8996	119, all the text-align keyword
right-to-left	8995	119, all the text-align keyword

No geometry moved in either direction.

The count is larger than the eleven converted rules because text-align inherits, so
every child reports the new keyword too. In right-to-left the changes read
right -> start and left -> end, which are the same rendering; in left-to-right they
read left -> start and right -> end.

rtl.css goes from 480 lines to 453.

Issues References (Fixes|Related|Closes)

Related to #7933.

---

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.