Base content style overrides

[cjg89]

This branch gives us a base set of content-related styles for Athena. It's starting to look much less Bootstrappy already, yay!
The diff is fairly long, but the main takeaways are bulleted below:

Tables:

• Removed border-top from all tables, except .table-bordered.
• Modified highlighted row background color, and added overrides for the .table-inverse tables to accommodate the changes.
• Updated .table-inverse border color to be gray instead of white for less harsh contrast.

Type:

• Added breakpoint-specific font size overrides on the body tag, headings, lead text, and blockquotes.
• Added font-family utility classes and some unique overrides to headings, lead text, and blockquotes to accommodate those font-family classes and help make font sizing across each supported font family more consistent.
• Updated display headings to use fluid font sizing (vw units) at the -xs breakpoint, so that they can be as large as possible while avoiding text overflow/horizontal scrollbars.
• Added stylized .heading-underline and .blockquote-quotation classes.
• Added utility classes for applying light and heavy font weights, as well as letter-spacing classes.

I'm using rem units here to match Bootstrap; there's some discussion on their reasoning for doing so here and here. One thing in particular to note: the responsive font size overrides added in this branch are all relative to the root font size--which is generally 16px--NOT to the font size of body. So, all rem-unit values are going to be relative to 16px (usually, depending on the browser/system). I opted to not utilize a responsive typography method (e.g. https://v4-alpha.getbootstrap.com/content/typography/#responsive-typography) because, despite the font sizes of headings, blockquotes, etc scaling up with the document font size, they tend to still need some breakpoint-specific adjustments, and adjusting them in relation to the current browser size adds an extra layer of complexity (compared to always adjusting sizes relative to ~16px).

Dividers:

• Added sizing and color variant classes for hr's

Other:

• Updated the default "primary" color to UCF gold, and moved the standard Bootstrap blue to a new "$brand-complementary" color.
• Added some other variables and made some modifications to existing values. Added some variables where they didn't exist previously when it didn't make sense for them to not exist (e.g. .table-inverse variant-specific variables)

[rjucf]

Instead of modifying the bootstrap _variables.scss would it make sense to create a _ucf_variables.scss included after the _variables.scss and overriding the bootstrap variables? This would make updating bootstrap easier especially since it is still alpha.

[cjg89]

It would make much more sense, but due to the way that sass variable inheritance works, it ends up being easier for us to just maintain our own custom copy of _variables.scss. Doing so makes sure that:
a) we always have access to Bootstrap variables when adding or modifying other variables (e.g. we don't run into issues with trying to use variables that aren't yet defined), and
b) changes we make to Bootstrap-specific variables cascade into other dependent variables.

Bootstrap actually started having custom-build users define a _custom.scss file that imported after Bootstrap's _variables.scss in an earlier alpha version, but ended up reverting the change due to variable overrides no longer working properly.