This implements the UI controls and information displays necessary to allow reviewing pull requests by stepping through commits individually.
Notable changes:
- Within the PR page, commit links now stay in the PR context by navigating to `{owner}/{repo}/pulls/{id}/commits/{sha}`
- When showing a single commit in the "Files changed" tab, the commit header containing commit message and metadata is displayed
- I dropped the existing buttons, since they make less sense to me in the PR context
- The SHA links to the separate, dedicated commit view
- "Previous"/"Next" buttons have been added to that header to allow stepping through commits
- Reviews can be submitted in "single commit" view
Talking points:
- The "Showing only changes from" banner made sense when that view was limited (e.g. review submit was disabled). Now that it's on par with the "all commits" view, and visually distinct due to the commit header, this banner could potentially be dropped.
Closes: #5670#5126#5671#2281#8084

## Checklist
The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).
### Tests
- I added test coverage for Go changes...
- [ ] in their respective `*_test.go` for unit tests.
- [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
- [ ] in `web_src/js/*.test.js` if it can be unit tested.
- [x] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7155
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Reviewed-by: Beowulf <beowulf@beocode.eu>
Co-authored-by: Lucas Schwiderski <lucas@lschwiderski.de>
Co-committed-by: Lucas Schwiderski <lucas@lschwiderski.de>
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

### Selecting a commit range

### Show changes of a single commit only

### Show changes of a commit range

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>