[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="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.

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

---
Conflict resolution: Trivial.
(cherry picked from commit 36887ed3921d03f1864360c95bd2ecf853bfbe72)
(cherry picked from commit f6c0c39f1a)

web_src/css/features/projects.css

@@ -22,34 +22,27 @@
   cursor: default;
 }
 
+.project-column .issue-card {
+  color: var(--color-text);
+}
+
 .project-column-header {
   display: flex;
   align-items: center;
   justify-content: space-between;
 }
 
-.project-column-header.dark-label {
-  color: var(--color-project-board-dark-label) !important;
-}
-
-.project-column-header.dark-label .project-column-title {
-  color: var(--color-project-board-dark-label) !important;
-}
-
-.project-column-header.light-label {
-  color: var(--color-project-board-light-label) !important;
-}
-
-.project-column-header.light-label .project-column-title {
-  color: var(--color-project-board-light-label) !important;
-}
-
 .project-column-title {
   background: none !important;
   line-height: 1.25 !important;
   cursor: inherit;
 }
 
+.project-column-title,
+.project-column-issue-count {
+  color: inherit !important;
+}
+
 .project-column > .cards {
   flex: 1;
   display: flex;
@@ -64,6 +57,8 @@
 
 .project-column > .divider {
   margin: 5px 0;
+  border-color: currentcolor;
+  opacity: .5;
 }
 
 .project-column:first-child {