Upgrade to bootstrap 5.2

[gravitystorm]

The upgrade exposed a problem with the way we were calling the mixins, so use named arguments instead. This is probably a better approach in any case.

Also remove a background declaration that is not required.

[gravitystorm]

There's something weird going on with sass in CI, so I've converted this to a draft while I go spelunking on that.

[gravitystorm]

There's something weird going on with sass in CI, so I've converted this to a draft while I go spelunking on that.

sass/sassc-rails#174 is the only mention I've found so far, doesn't hint at any workarounds.

[Firefishy]

sass/sassc-ruby#233 worked for me, I cannot comment if it is a pretty or the "right" solution.

[Firefishy]

There are also relevant comments here: sass/sassc-ruby#220

[gravitystorm]

@Firefishy great, thanks for the links.

Since this isn't urgent, and figuring out "what to do with sass" is a much bigger and more complex situation than I expected, I'm going to close this PR for now. I'll revisit the bootstrap 5.2 upgrade in a few weeks/months and see what the wider state of play is by that point.