[PORT] gitea##30237: Fix and rewrite contrast color calculation, fix project-related bugs

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.

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

---
Conflict resolution: Trivial.
(cherry picked from commit 36887ed3921d03f1864360c95bd2ecf853bfbe72)

modules/templates/helper.go

@@ -53,13 +53,13 @@ func NewFuncMap() template.FuncMap {
 		"JsonUtils":   NewJsonUtils,
 
 		// -----------------------------------------------------------------
-		// svg / avatar / icon
+		// svg / avatar / icon / color
 		"svg":           svg.RenderHTML,
 		"EntryIcon":     base.EntryIcon,
 		"MigrationIcon": MigrationIcon,
 		"ActionIcon":    ActionIcon,
-
-		"SortArrow": SortArrow,
+		"SortArrow":     SortArrow,
+		"ContrastColor": util.ContrastColor,
 
 		// -----------------------------------------------------------------
 		// time / number / format