Add/utilize fontWeight* variables

[jaridmargolin]

Right now, semantic-ui utilizes normal and bold values for setting font-weight's throughout the codebase. By default, browsers will map the bold -> 700 and normal -> 400 (this is somewhat of a simplification). For the most part, this strategy works.

I have, however, rubbed up against a requirement where I need to remap these values to different numerical weights (500 and 300). The @font-face declarations come down from a 3rd party service and I have no means of adjusting how they are defined. Right now, the only way I was able to accomplish this within semantic-ui, was to declare variables/overrides for all occasions where font-weight is set. It was a horribly tedious and the end solution feels very brittle as the code base changes (created nearly 30 files to accomplish this).

This PR adds support for dynamically setting the font weights used throughout semantic UI by defining three new variables, fontWeightBold, fontWeightNormal, and fontWeightIcon.

I believe this would properly address the concern brought up in #2846

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As for testing these changes, I have built the dist files locally and manually previewed examples/theming.html. All UI appeared normal, however, I am uncertain if there is a more robust solution for verifying UI updates.

[jaridmargolin]

@jlukic - I understand how difficult it can be to manage an open source project of this scale, so I hope nothing I say here comes off as offensive. I notice there is ~160 open PR's. I'm wondering what the likilhood is that this PR will get reviewed in a timely manner.

Is there anything I can do to help push it through? or at least get closure regarding wether or not there is interest in merging.

Thank you for you all of your hard work. Appreciate anything you can do here :)

[jlukic]

This is pretty low friction update.

I think I'd prefer to do this as a single global variable @bold and @normal defined in site.variables, and not modify any component variables outside of replacing these values.

This is similar to how we handle px size in em with @15px etc

To reconfigure on a component level you can just redefine @bold: 600 inside the site or theme button.variables since all our variables are scoped per component.

[jlukic]

Added in #6171