add Bootstrap 5 support

[guruofgentoo]

Switch to Bootstrap 5 by default, add two config keys to make selecting bootstrap version for template support a bit easier.

• select bootstrap version via KEG_BOOTSTRAP_MAJOR_VERSION config (default 5)
• select whether bootstrap is used at all via KEG_TEMPLATE_FLAVOR config (default bootstrap)
• use tomselect for bootstrap 5+, in place of select2 (to remove jQuery dependency)

refs #188

BC BREAK Apps that want to stay on Bootstrap 4 will need to set KEG_BOOTSTRAP_MAJOR_VERSION to 4. Apps could also begin extending the horizontal.html form template instead of horizontal-b4.html, but that change is not required.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.17%. Comparing base (aa60cd6) to head (8e47e4c).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   96.17%   96.17%           
=======================================
  Files          33       33           
  Lines        3840     3840           
=======================================
  Hits         3693     3693           
  Misses        147      147           

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[rsyring]

@guruofgentoo any reason this didn't end up getting merged?

[guruofgentoo]

IIRC it was just due to the BC break at the time, and then fell off the radar. I had forgotten this was out there. We should bring it in though - it makes dealing with the different Bootstrap versions much more explicit.