Create shiny sass variable default files specific to Bootstrap Version

[gadenbuie]

I reverted inst/www/shared/shiny_scss/bootstrap.scss back to its state before the BS 5.3 updates. Those updates tried to support BS 3, 4 and 5.3 in the same bootstrap.scss file.

Then I created variants for each major version of Bootstrap. This allows us to confidently use features of each version of Bootstrap without a lot of checking for variable existence or having to backport Sass variables for older versions of Bootstrap.

The original bootstrap.scss had a small amount of backporting to simultaneously support BS3 and BS4; I removed the backports in each version.

[cpsievert]

LGTM, thanks. Would also be great to have a short explanation of what this PR changes in terms of default Bootstrap 5 styles. Please also update news.

[cpsievert]

@gadenbuie it'd be great to get this in fairly soon, since it seems like it could impact testing screenshots.

To that end, I am a bit surprised that inst/www/shared/shiny.min.css hasn't changed as a result of these changes, so before merging, let's make sure that is updated (if need be)

[gadenbuie]

To that end, I am a bit surprised that inst/www/shared/shiny.min.css hasn't changed as a result of these changes, so before merging, let's make sure that is updated (if need be)

Is that file compiled against BS3? In which case it'd be a good thing we don't see changes.

[gadenbuie]

@cpsievert Sorry, I forgot this was waiting on changes from me. I've removed the comments and added a news item.

[cpsievert]

Is that file compiled against BS3? In which case it'd be a good thing we don't see changes.

Ah, good point, thanks!