AccessKit: Make links blue consistently

[marcustyphoon]

Description

This fixes instances where AccessKit's "Make links in posts blue" feature does not work (see linked issue), as well as choosing a color from the palette that is a) more often actually blue and b) sort of arguably more correct... on a larger number of palettes? (see comments below)

Resolves #1135.

Testing steps

Test the relevant AccessKit option on:

links that go to anywhere on a Tumblr domain, mentions... or links in posts when they're reblogged on the staff blog or Matt's blog

Test the relevant AccessKit option on various palettes; see the comments below.

[marcustyphoon]

As an aside: should this use accent or blue?

Palettes where it doesn't really matter:

• true blue
• dark mode
• low contrast
• cybernetic
• pride
• ghost

Palettes where I prefer blue:

• cement
• canary
• vampire
• pumpkin
• ["decision 2016" from palettes for tumblr]

Palettes where I prefer accent:

• snow bright
• goth rave

To play with this, I used

article ${keyToCss('textBlock')} a[target="_blank"] {
  color: rgba(var(--accent));
}
article ${keyToCss('textBlock')}:first-child a[target="_blank"] {
  color: rgba(var(--blue));
}

[marcustyphoon]

Notably, blue is used to highlight tabs in the notes view container, while accent is used to highlight comments that are by the original post author. Screenshot taken with the mention link highlighted blue.