Fixed #34038 -- Improve link readability and identifiability

[CLWilliam]

To improve the readability of links in the Django admin:

• The default colour has been slightly darkened in light mode (-1% lightness in HSL colour space)
• The links are underlined on hover for all default links (add/create links, breadcrumbs...) to be more identifiable for people with colour vision deficiencies.
• Links in the form rows have been underlined by default as - since they bend into the help text - they are very difficult to distinguish from plain text.

[knyghty]

Sorry for the delay. This looks good, but I think we can remove the hover style, I think it looks better without and doesn't provide much value IMO.

[felixxm]

@CLWilliam Do you have time to keep working on this? (see #16101 (review))

[CLWilliam]

@felixxm it's on the menu for this evening!

[felixxm]

@thibaudcolas Can you take a look?

[felixxm]

The new color have enough contrast when the background is white. Unfortunately --link-fg is also used with yellow (#ffffcc) and gray backgrounds (#f8f8f8). We should make it even darker to meet AA standards, e.g. #417893 (\cc @knyghty.)

[felixxm]

Closing due to inactivity.