Allow force push to protected branches (#28086)

Fixes #22722 

### Problem
Currently, it is not possible to force push to a branch with branch
protection rules in place. There are often times where this is necessary
(CI workflows/administrative tasks etc).

The current workaround is to rename/remove the branch protection,
perform the force push, and then reinstate the protections.

### Solution
Provide an additional section in the branch protection rules to allow
users to specify which users with push access can also force push to the
branch. The default value of the rule will be set to `Disabled`, and the
UI is intuitive and very similar to the `Push` section.

It is worth noting in this implementation that allowing force push does
not override regular push access, and both will need to be enabled for a
user to force push.

This applies to manual force push to a remote, and also in Gitea UI
updating a PR by rebase (which requires force push)

This modifies the `BranchProtection` API structs to add:
- `enable_force_push bool`
- `enable_force_push_whitelist bool`
- `force_push_whitelist_usernames string[]`
- `force_push_whitelist_teams string[]`
- `force_push_whitelist_deploy_keys bool`

### Updated Branch Protection UI:

<img width="943" alt="image"
src="7491899c-d816-45d5-be84-8512abd156bf">

### Pull Request `Update branch by Rebase` option enabled with source
branch `test` being a protected branch:


![image](e018e6e9-b7b2-4bd3-808e-4947d7da35cc)
<img width="1038" alt="image"
src="57ead13e-9006-459f-b83c-7079e6f4c654">

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/pull/update.go

@@ -117,27 +117,25 @@ func IsUserAllowedToUpdate(ctx context.Context, pull *issues_model.PullRequest,
 		return false, false, err
 	}
 
-	// can't do rebase on protected branch because need force push
-	if pb == nil {
-		if err := pr.LoadBaseRepo(ctx); err != nil {
-			return false, false, err
+	if err := pr.LoadBaseRepo(ctx); err != nil {
+		return false, false, err
+	}
+	prUnit, err := pr.BaseRepo.GetUnit(ctx, unit.TypePullRequests)
+	if err != nil {
+		if repo_model.IsErrUnitTypeNotExist(err) {
+			return false, false, nil
 		}
-		prUnit, err := pr.BaseRepo.GetUnit(ctx, unit.TypePullRequests)
-		if err != nil {
-			if repo_model.IsErrUnitTypeNotExist(err) {
-				return false, false, nil
-			}
-			log.Error("pr.BaseRepo.GetUnit(unit.TypePullRequests): %v", err)
-			return false, false, err
-		}
-		rebaseAllowed = prUnit.PullRequestsConfig().AllowRebaseUpdate
+		log.Error("pr.BaseRepo.GetUnit(unit.TypePullRequests): %v", err)
+		return false, false, err
 	}
 
-	// Update function need push permission
+	rebaseAllowed = prUnit.PullRequestsConfig().AllowRebaseUpdate
+
+	// If branch protected, disable rebase unless user is whitelisted to force push (which extends regular push)
 	if pb != nil {
 		pb.Repo = pull.BaseRepo
-		if !pb.CanUserPush(ctx, user) {
-			return false, false, nil
+		if !pb.CanUserForcePush(ctx, user) {
+			rebaseAllowed = false
 		}
 	}