css: rework color scheme to use new color palette.

Add a new color palette and rework all site colors (both light mode and dark mode) to
use the new palette.

This ensures that colors are used consistently, from a carefully designed color palette,
instead of being chosen at random.

Before, colors in light mode were chosen on an ad-hoc basis, which resulted in a lot of
random colors and inconsistent design.

The new palette has 7 hues: red, orange, yellow, green, blue, azure (a lighter blue), and
purple. There's also a greyscale. Each hue has 10 shades of brightness, which (including
grey) gives us 80 total colors.

Colors are named like this:

    var(--red-0);    /* very light red */
    var(--red-2);    /* light red */
    var(--red-5);    /* medium red */
    var(--red-7);    /* dark red */
    var(--red-9);    /* very dark red */
    var(--green-7);  /* dark green */
    var(--blue-5);   /* medium blue */
    var(--purple-3); /* light purple */
                     /* etc */

The color palette is designed to meet the following criteria:

* To have close equivalents to the main colors used in the old color scheme,
  especially tag colors, so that changes to major colors are minimized.
* To produce a set of colors that can be used as as main text colors, as background
  colors, and as accent colors, both in light mode and dark mode.
* To ensure that colors at the same brightness level have the same perceived brightness.
  Green-4, blue-4, red-4, purple-4, etc should all have the same brightness and contrast
  ratios. This way colors look balanced. This is actually a difficult problem, because human
  color perception is non-linear, so you can't just scale brightness values linearly.

There's a color palette test page at https://danbooru.donmai/static/colors

Notable changes to colors in light mode:

* Username colors are the same as tag colors.
* Copyright tags are a deeper purple.
* Builders are a deeper purple (fixes #4626).
* Moderators are green.
* Gold users are orange.
* Parent borders are a darker green.
* Child borders are a darker orange.
* Unsaved notes have a thicker red border.
* Selected notes have a thicker blue (not green) border.

app/logical/color_palette.rb

@@ -0,0 +1,46 @@
+module ColorPalette
+  module_function
+
+  HUES = {
+    grey: 265,
+    red: 12,
+    orange: 37,
+    yellow: 68,
+    green: 130,
+    azure: 242,
+    blue: 257,
+    purple: 282,
+  }
+
+  SATURATIONS = {
+    grey:   [17.5]*10,
+    red:    [100, 80, 100, 100, 100, 90, 100, 90, 75, 50],
+    yellow: [60, 60, 60, 100, 100, 100, 100, 100, 100, 90],
+    green:  [40, 40, 50, 90, 100, 100, 100, 100, 100, 100],
+    purple: [100, 70, 90, 100, 100, 100, 100, 100, 80, 80],
+  }
+
+  LIGHTNESSES = [
+    97.0, # 0
+    92.0, # 1
+    84.0, # 2
+    70.5, # 3
+    61.0, # 4
+    51.2, # 5
+    40.0, # 6
+    28.0, # 7
+    19.0, # 8
+    12.0, # 9
+  ]
+
+  def css_rules
+    HUES.flat_map do |name, hue|
+      LIGHTNESSES.each_with_index.map do |lightness, i|
+        saturation = SATURATIONS.dig(name, i) || 100
+        hex = Hsluv.hsluv_to_hex(hue, saturation, lightness)
+
+        "--#{name}-#{i}: #{hex}; /* hsluv(#{hue}, #{saturation}, #{lightness}) */"
+      end
+    end.join("\n")
+  end
+end