Add commits dropdown in PR files view and allow commit by commit review (#25528)

This PR adds a new dropdown to select a commit or a commit range
(shift-click like github) of a Pull Request.
After selection of a commit only the changes of this commit will be shown.
When selecting a range of commits the diff of this range is shown.

This allows to review a PR commit by commit or by viewing only commit ranges.
The "Show changes since your last review" mechanism github uses is implemented, too.
When reviewing a single commit or a commit range the "Viewed" functionality is disabled.

## Screenshots

### The commit dropdown

![image](https://github.com/go-gitea/gitea/assets/51889757/0db3ae62-1272-436c-be64-4730c5d611e3)

### Selecting a commit range

![image](https://github.com/go-gitea/gitea/assets/51889757/ad81eedb-8437-42b0-8073-2d940c25fe8f)

### Show changes of a single commit only

![image](https://github.com/go-gitea/gitea/assets/51889757/6b1a113b-73ef-4ecc-adf6-bc2340bb8f97)

### Show changes of a commit range

![image](https://github.com/go-gitea/gitea/assets/51889757/6401b358-cd66-4c09-8baa-6cf6177f23a7)


Fixes https://github.com/go-gitea/gitea/issues/20989
Fixes https://github.com/go-gitea/gitea/issues/19263

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

web_src/css/base.css

@@ -633,6 +633,11 @@ a.label,
   color: var(--color-text-light-2);
 }
 
+.ui.dropdown > .text > .description,
+.ui.dropdown .menu > .item > .description {
+  color: var(--color-text-light-2);
+}
+
 .ui.list .list > .item .header,
 .ui.list > .item .header {
   color: var(--color-text-dark);