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/issue-list.css

@@ -34,23 +34,6 @@
   }
 }
 
-#issue-list .flex-item-title .labels-list {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 0.25em;
-}
-
-#issue-list .flex-item-title .labels-list a {
-  display: flex;
-  text-decoration: none;
-}
-
-#issue-list .flex-item-title .labels-list .label {
-  padding: 0 6px;
-  margin: 0;
-  min-height: 20px;
-}
-
 #issue-list .flex-item-body .branches {
   display: inline-flex;
 }