Set default link color to blue-500

What does this MR do and why?

Make all links $blue-500 by default, to match gl-link color.

1. set $link-color variable (which bootstrap uses)
2. remove our overrides that were setting link color to $blue-600

Fixes bug where gl-links look weird when near <a> tags

Look at these two links

Even more noticeable when hovering

Screenshots or screen recordings

Light mode, changes from #1068bf to #1f75cb

Dark mode, changes from #63a6e9 to #428fdc

How to set up and validate locally

1. Browse some links
2. Soak up the contrast and color balance

MR acceptance checklist

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