Support only minimum version of Bootstrap

[asbates]

Limit support to a minimum version of Bootstrap. For example, only support Bootstrap 4+.

The default of Bootstrap 3 is outdated and apps would look much nicer by just using a newer version, even without style changes. Supporting bootstrap 3 is also more cumbersome because e.g. it doesn't have CSS variables. This would also make it easier to have continuity of styling a la #735.

If we do this, we would have teal set the minimum version, maybe through options(teal.bs_theme). We would of course allow users to override this if they desire.

@donyunardi What do you think about this?

[donyunardi]

I like the idea but curious how this will impact any legacy teal app. My gut feeling is saying there won't be much impact.

We should investigate further. Adding to backlog.