Fix and rewrite contrast color calculation, fix project-related bugs (#30237)

1. The previous color contrast calculation function was incorrect at
least for the `#84b6eb` where it output low-contrast white instead of
black. I've rewritten these functions now to accept hex colors and to
match GitHub's calculation and to output pure white/black for maximum
contrast. Before and after:
<img width="94" alt="Screenshot 2024-04-02 at 01 53 46"
src="00b39e15-a377-4458-95cf-ceec74b78228"><img
width="90" alt="Screenshot 2024-04-02 at 01 51 30"
src="1677067a-8d8f-47eb-82c0-76330deeb775">

2. Fix project-related issues:

- Expose the new `ContrastColor` function as template helper and use it
for project cards, replacing the previous JS solution which eliminates a
flash of wrong color on page load.
- Fix a bug where if editing a project title, the counter would get
lost.
- Move `rgbToHex` function to color utils.

@HesterG fyi

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/templates/util_render.go

@@ -123,16 +123,10 @@ func RenderIssueTitle(ctx context.Context, text string, metas map[string]string)
 func RenderLabel(ctx context.Context, locale translation.Locale, label *issues_model.Label) template.HTML {
 	var (
 		archivedCSSClass string
-		textColor        = "#111"
+		textColor        = util.ContrastColor(label.Color)
 		labelScope       = label.ExclusiveScope()
 	)
 
-	r, g, b := util.HexToRBGColor(label.Color)
-	// Determine if label text should be light or dark to be readable on background color
-	if util.UseLightTextOnBackground(r, g, b) {
-		textColor = "#eee"
-	}
-
 	description := emoji.ReplaceAliases(template.HTMLEscapeString(label.Description))
 
 	if label.IsArchived() {
@@ -153,7 +147,7 @@ func RenderLabel(ctx context.Context, locale translation.Locale, label *issues_m
 
 	// Make scope and item background colors slightly darker and lighter respectively.
 	// More contrast needed with higher luminance, empirically tweaked.
-	luminance := util.GetLuminance(r, g, b)
+	luminance := util.GetRelativeLuminance(label.Color)
 	contrast := 0.01 + luminance*0.03
 	// Ensure we add the same amount of contrast also near 0 and 1.
 	darken := contrast + math.Max(luminance+contrast-1.0, 0.0)
@@ -162,6 +156,7 @@ func RenderLabel(ctx context.Context, locale translation.Locale, label *issues_m
 	darkenFactor := math.Max(luminance-darken, 0.0) / math.Max(luminance, 1.0/255.0)
 	lightenFactor := math.Min(luminance+lighten, 1.0) / math.Max(luminance, 1.0/255.0)
 
+	r, g, b := util.HexToRBGColor(label.Color)
 	scopeBytes := []byte{
 		uint8(math.Min(math.Round(r*darkenFactor), 255)),
 		uint8(math.Min(math.Round(g*darkenFactor), 255)),