Rename project board -> column to make the UI less confusing (#30170)

This PR split the `Board` into two parts. One is the struct has been
renamed to `Column` and the second we have a `Template Type`.

But to make it easier to review, this PR will not change the database
schemas, they are just renames. The database schema changes could be in
future PRs.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: yp05327 <576951401@qq.com>

web_src/css/features/projects.css

@@ -7,7 +7,7 @@
 }
 
 .project-column {
-  background-color: var(--color-project-board-bg) !important;
+  background-color: var(--color-project-column-bg) !important;
   border: 1px solid var(--color-secondary) !important;
   margin: 0 0.5rem !important;
   padding: 0.5rem !important;

web_src/css/themes/theme-gitea-dark.css

@@ -216,7 +216,7 @@
   --color-expand-button: #2f363d;
   --color-placeholder-text: var(--color-text-light-3);
   --color-editor-line-highlight: var(--color-primary-light-5);
-  --color-project-board-bg: var(--color-secondary-light-2);
+  --color-project-column-bg: var(--color-secondary-light-2);
   --color-caret: var(--color-text); /* should ideally be --color-text-dark, see #15651 */
   --color-reaction-bg: #e8f3ff12;
   --color-reaction-hover-bg: var(--color-primary-light-4);

web_src/css/themes/theme-gitea-light.css

@@ -216,7 +216,7 @@
   --color-expand-button: #cfe8fa;
   --color-placeholder-text: var(--color-text-light-3);
   --color-editor-line-highlight: var(--color-primary-light-6);
-  --color-project-board-bg: var(--color-secondary-light-4);
+  --color-project-column-bg: var(--color-secondary-light-4);
   --color-caret: var(--color-text-dark);
   --color-reaction-bg: #0000170a;
   --color-reaction-hover-bg: var(--color-primary-light-5);