Ignore percentage values for compare in maps

[romanlex]

All conversation about it in #24061 (comment)
I think this is the only solution

Short:
We cant compare percantage values with absolute values in map, but many people want use % in $container-max-widths

Fixes #24061

[andresgalante]

Thanks a lot @romanlex, this works and it'll give the chance to author to pick % or px.

There has been back and forwards and discussions for a very long time about the units of the breakpoints and the fact that we don't allow them to be rem or em.

I'll like @patrickhlauke and @mdo to decide what are the implications of allowing % for breakpoints.

Also if we merge this, there should be also a change in the docs here:
http://getbootstrap.com/docs/4.1/layout/grid/#grid-options

[Paladinium]

@andresgalante , @patrickhlauke @mdo : Would be really nice to see this change in one of the next minor releases...

[errge]

If I'm not mistaken, this only fixed % warning, if you use percentage at the biggest unit.

E.g. the bug report was: sm 500, md 700, lg 850, xl 100%.

But I have a use case, where I have only two categories: small: 80%, big: 720px. So big is never too wide for reading, but small always has a nice padding from the screen (80% width). This causes a warning still, because not the current unit is "%", but the previous unit. Can we also ignore the warning in case of unit($prev-num) == "%"?

[XhmikosR]

@errge you should make a new issue