Refactor project templates (#26448)

This PR refactors a bunch of projects-related code, mostly the
templates.
The following things were done:
  - rename boards to columns in frontend code
  - use the new `ctx.Locale.Tr` method
  - cleanup template, remove useless newlines, classes, comments
  - merge org-/user and repo level project template together
    - move "new column" button into project toolbar
- move issue card (shared by projects and pinned issues) to shared
template, remove useless duplicated styles
- add search function to projects (to make the layout more similar to
milestones list where it is inherited from 😆)
  - maybe more changes I forgot I've done 😆 

Closes #24893

After:
![Bildschirmfoto vom 2023-08-10
23-02-00](https://github.com/go-gitea/gitea/assets/47871822/cab61456-1d23-4373-8163-e567f1b3b5f9)
![Bildschirmfoto vom 2023-08-10
23-02-26](https://github.com/go-gitea/gitea/assets/47871822/94b55d60-5572-48eb-8111-538a52d8bcc6)
![Bildschirmfoto vom 2023-08-10
23-02-46](https://github.com/go-gitea/gitea/assets/47871822/a0358f4b-4e05-4194-a7bc-6e0ecba5a9b6)

---------

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/repo/issue-card.css

@@ -0,0 +1,21 @@
+.issue-card {
+  display: flex;
+  flex-direction: column;
+  align-items: start;
+  border-radius: var(--border-radius);
+  padding: 8px 10px;
+  border: 1px solid var(--color-secondary);
+  background: var(--color-card);
+}
+
+.issue-card-icon,
+.issue-card-unpin {
+  margin-top: 1px;
+  flex-shrink: 0;
+}
+
+.issue-card-title {
+  flex: 1;
+  font-size: 18px;
+  margin-left: 4px;
+}