Add RTL style overrides, ensure RTL CSS is loaded on RTL sites

[ryelle]

Fixes #2483. This updates the style.css registration to create an RTL replacement on RTL sites — this means the style-index-rtl.css file will be used instead. This PR also includes a few overrides specific to RTL sites, in the _rtl.scss file.

The issues were broadly with the Meeting Calendar plugin, Sensei, and hardcoded styles on blocks.

• The meeting calendar could probably be fixed by updating the wp-scripts package there, which would create RTL styles (but the 15 -> 27 version jump would probably cause other issues).
• Sensei's RTL support is limited, see RTL Design Support Automattic/sensei#1836, so there are overrides in the _rtl.scss file.
• The Learning Pathway Card sets the padding on the group block directly, and since it's loaded via a template, the convert_inline_style_to_rtl never runs to flip those values (hopefully the the_block_template_html filter will be added in 6.7, which will be a better place to run that filter) — in any case, I've added some CSS to _rtl.scss for that case as well.

Screenshots

	Before	After
Lesson page
Learning Pathway homepage
Online workshops
Course homepage
Quiz

[adamwoodnz]

The My Courses filter still need to be switched to the opposite side

[adamwoodnz]

Quiz radio inputs look good but checkboxes are misaligned, see https://learn.wordpress.org/quiz/how-to-use-headings-for-accessibility-and-seo/

[adamwoodnz]

The images in the Learning Pathway cards get cropped from the wrong side on smaller screens (screenshots are at iPad landscape size)

LTR	RTL

And on the homepage some borders need switching

[adamwoodnz]

Looking much better, but I found a few more things sorry

[ryelle]

The My Courses filter still need to be switched to the opposite side

This looks correct for me:

And on the homepage some borders need switching

I can't replicate this, borders look fine to me:

Do you have an out of date wporg-parent on your sandbox, maybe? These are both things that the RTL filters should fix.

Quizzes with checkboxes have been fixed (6d12c9e)

And I've flipped the image alignment on learning pathways cards for RTL sites (c53f67c)

[adamwoodnz]

LGTM now except for the border styles. I've updated all on my sandbox and still seeing them. The culprit seems to be https://learn.wordpress.org/wp-content/plugins/gutenberg/build/block-library/common-rtl.css?ver=18.8.0

If you're not seeing it then maybe ignore.

[ryelle]

Aha, I managed to trigger it by preventing inline styles — I was seeing this as an inline style, but it's using the unflipped LTR version here (that seems like a different problem 😬 )

This style should not be flipped in the RTL stylesheet, and is a bug in gutenberg- by setting the border-right-style it inherits default a width & color; and since the left style is not set, it's not visible.

This can be worked around by setting the right style (to make the inline-end border visible) and the left width 0 (to hide the inline-start border). Left/right are flipped in the RTL function in the parent theme, so it's easier to talk in logical property direction 🙂