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>

models/fixtures/issue.yml

@@ -304,3 +304,20 @@
   created_unix: 946684830
   updated_unix: 978307200
   is_locked: false
+
+-
+  id: 19
+  repo_id: 58
+  index: 1
+  poster_id: 2
+  original_author_id: 0
+  name: issue for pr
+  content: content
+  milestone_id: 0
+  priority: 0
+  is_closed: false
+  is_pull: true
+  num_comments: 0
+  created_unix: 946684830
+  updated_unix: 978307200
+  is_locked: false

models/fixtures/pull_request.yml

@@ -76,3 +76,16 @@
   base_branch: master
   merge_base: 2a47ca4b614a9f5a
   has_merged: false
+
+-
+  id: 7
+  type: 0 # gitea pull request
+  status: 2 # mergable
+  issue_id: 19
+  index: 1
+  head_repo_id: 58
+  base_repo_id: 58
+  head_branch: branch1
+  base_branch: main
+  merge_base: cbff181af4c9c7fee3cf6c106699e07d9a3f54e6
+  has_merged: false

models/fixtures/repo_unit.yml

@@ -607,3 +607,33 @@
   repo_id: 52
   type: 1
   created_unix: 946684810
+
+-
+  id: 91
+  repo_id: 58
+  type: 1
+  created_unix: 946684810
+
+-
+  id: 92
+  repo_id: 58
+  type: 2
+  created_unix: 946684810
+
+-
+  id: 93
+  repo_id: 58
+  type: 3
+  created_unix: 946684810
+
+-
+  id: 94
+  repo_id: 58
+  type: 4
+  created_unix: 946684810
+
+-
+  id: 95
+  repo_id: 58
+  type: 5
+  created_unix: 946684810

models/fixtures/repository.yml

@@ -1662,3 +1662,34 @@
   is_private: false
   status: 0
   num_issues: 0
+
+-
+  id: 58 # org public repo
+  owner_id: 2
+  owner_name: user2
+  lower_name: commitsonpr
+  name: commitsonpr
+  default_branch: main
+  num_watches: 0
+  num_stars: 0
+  num_forks: 0
+  num_issues: 0
+  num_closed_issues: 0
+  num_pulls: 1
+  num_closed_pulls: 0
+  num_milestones: 0
+  num_closed_milestones: 0
+  num_projects: 0
+  num_closed_projects: 0
+  is_private: false
+  is_empty: false
+  is_archived: false
+  is_mirror: false
+  status: 0
+  is_fork: false
+  fork_id: 0
+  is_template: false
+  template_id: 0
+  size: 0
+  is_fsck_enabled: true
+  close_issues_via_commit_in_any_branch: false

models/fixtures/user.yml

@@ -66,7 +66,7 @@
   num_followers: 2
   num_following: 1
   num_stars: 2
-  num_repos: 13
+  num_repos: 14
   num_teams: 0
   num_members: 0
   visibility: 0