Fix font stack output

[thisisdano]

This fixes a problem where we might be outputting font stacks with a leading comma or using a redundant display name at the start of the stack.

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We have four internal font stacks that we can assign to to typefaces, composed of common fallbacks installed on most systems:

$font-stack-system: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';
$font-stack-georgia: 'Georgia', 'Cambria', 'Times New Roman', 'Times', serif;
$font-stack-helvetica: 'Helvetica Neue', 'Helvetica', 'Roboto', 'Arial', sans-serif;
$font-stack-monospace: 'Bitstream Vera Sans Mono', 'Consolas', 'Courier', monospace;

Plus, users can define custom stacks for font types in settings:

$theme-font-cond-custom-stack:  false !default;
$theme-font-icon-custom-stack:  false !default;
$theme-font-lang-custom-stack:  false !default;
$theme-font-mono-custom-stack:  false !default;
$theme-font-sans-custom-stack:  false !default;
$theme-font-serif-custom-stack: false !default;

The output font stack is the display name of the typeface plus the value of either the system-assigned stack or the user's custom stack.

For example, Source Sans Pro gets the $font-stack-helvetica stack by default and is output as

font-family: #{display-name}, #{font-stack};

But the builder should not put a display name at the top of the stack in a couple of scenarios:

• If the user is using system fonts
• If the typeface is the same as the stack (i.e. helvetica or georgia) and no custom stack is defined

Now the stack builder function is smart enough to do that. It will withhold the leading display name if the display name is null (in the case of 'system') or if the assigned typeface is one of the common system fonts we're already using as a stack (in the case of 'georgia' or 'helvetica').

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Fixes #2944