Add "Allow edits from maintainer" feature (#18002)

Adds a feature [like GitHub has](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork) (step 7).
If you create a new PR from a forked repo, you can select (and change later, but only if you are the PR creator/poster) the "Allow edits from maintainers" option.
Then users with write access to the base branch get more permissions on this branch:
* use the update pull request button
* push directly from the command line (`git push`)
* edit/delete/upload files via web UI
* use related API endpoints

You can't merge PRs to this branch with this enabled, you'll need "full" code write permissions.

This feature has a pretty big impact on the permission system. I might forgot changing some things or didn't find security vulnerabilities. In this case, please leave a review or comment on this PR.

Closes #17728

Co-authored-by: 6543 <6543@obermui.de>

routers/web/repo/compare.go

@@ -398,6 +398,7 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 	}
 
 	ctx.Data["HeadRepo"] = ci.HeadRepo
+	ctx.Data["BaseCompareRepo"] = ctx.Repo.Repository
 
 	// Now we need to assert that the ctx.Doer has permission to read
 	// the baseRepo's code and pulls
@@ -436,6 +437,7 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 			ctx.NotFound("ParseCompareInfo", nil)
 			return nil
 		}
+		ctx.Data["CanWriteToHeadRepo"] = permHead.CanWrite(unit.TypeCode)
 	}
 
 	// If we have a rootRepo and it's different from: