Also, improve contrast of comments

Signed-off-by: Maxim Belkin <maxim.belkin@gmail.com>

Use the same colors but apply 12.5% opacity.

Solution boxes use 30% opacity

Pull Request: carpentries/styles#388

Clarify wording of PR & issue templates

I see lots of contributions where none of this placeholder text has
been deleted, but also quite a lot where the first line has been deleted,
but the 'ext below' hasn't (exactly in accordance with the current
instructions).

links.md: lesson-specific links

Signed-off-by: Maxim Belkin <maxim.belkin@gmail.com>

Pull Request: carpentries/styles#374

aio.md: fix indentation and variable declaration

ISSUE_TEMPLATE.md: Nudge contributors to specify episodes unambigously

See https://github.com/swcarpentry/r-novice-inflammation/issues/401 > 3.

Fix https://github.com/carpentries/workshop-template/issues/540

See apple.com/macos