index: migrate from label to badge (Bootstrap 3 -> 4/5)

[jrchamp]

label was in Bootstrap 3. Since Bootstrap 4 (theme boost; Moodle 3.2+), it's been badge and we don't need/want the custom CSS. The suggested change should work in Bootstrap 4.0 - 5.3+ (specifically Moodle 3.2 - 4.6+).

Fixes #601

[smbader]

I like the change, supporting back to 3.2 is great.

As mentioned in our meeting, there's a little bit of contrast back and forth between versions that we need to work out.

[jrchamp]

Thanks y'all for testing these with me!

Even with the backward/forward compatibility, badge bg-info text-dark was not sufficient contrast for WCAG 2.x AA in some versions. Forcing text-dark also had the drawback of not working well in "dark mode" themes. So if we can't use text-dark, then Bootstrap 5.0 and 5.1 will need to use a background style that has sufficient contrast with the default text color. That leaves "primary", "secondary", "success", "danger" and "dark". primary and secondary are already used for buttons to Join or Cancel. success did not have sufficient contrast in some of the versions that we tested. danger is specifically designed to scare/alert people, so that felt incorrect. Thus, dark is the only option that has sufficient contrast, isn't designed to scare people, and does not conflict with the mental model of action buttons.

[smbader]

A good choice for both visibility and backwards compatibility