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="https://github.com/go-gitea/gitea/assets/115237/00b39e15-a377-4458-95cf-ceec74b78228"><img
width="90" alt="Screenshot 2024-04-02 at 01 51 30"
src="https://github.com/go-gitea/gitea/assets/115237/1677067a-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>

web_src/css/repo.css

@@ -2273,8 +2273,21 @@
   height: 0.5em;
 }
 
+.labels-list {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.25em;
+}
+
+.labels-list a {
+  display: flex;
+  text-decoration: none;
+}
+
 .labels-list .label {
-  margin: 2px 0;
+  padding: 0 6px;
+  margin: 0 !important;
+  min-height: 20px;
   display: inline-flex !important;
   line-height: 1.3; /* there is a `font-size: 1.25em` for inside emoji, so here the line-height needs to be larger slightly */
 }