`Locales can opt-in` should say `opt in` instead

[flodolo]

Opt-in in a noun, not a verb.

This requires fixing the Django templates, but also updating documentation. Bonus point for searching for the same typo across the code base.

This is a good first bug for new contributors.

[zahradm]

Hi, I want to work on it!

[mathjazz]

Assigned :)

[flodolo]

Please wait a few hours to work on this, we're finalizing a change that will impact this work.

[zahradm]

Thanks for the heads-up! I'll hold off and wait for the all-clear. I'll check back in a few hours. Just to confirm—should I watch this issue for an update, or will you ping me here when it's ready? @flodolo

[flodolo]

I'll ping there this is ready.