Allow configuring default PR base branch (fixes #36412) (#36425)

This adds a per-repository default PR base branch and wires it through
PR entry points. It updates compare links and recently pushed branch
prompts to respect the configured base branch, and prevents auto-merge
cleanup from deleting the configured base branch on same-repo PRs.

## Behavior changes
- New PR compare links on repo home/issue list and branch list honor the
configured default PR base branch.
- The "recently pushed new branches" prompt now compares against the
configured base branch.
- Auto-merge branch cleanup skips deleting the configured base branch
(same-repo PRs only).

---------

Signed-off-by: Louis <116039387+tototomate123@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/issue.go

@@ -109,11 +109,6 @@ func MustAllowPulls(ctx *context.Context) {
 		ctx.NotFound(nil)
 		return
 	}
-
-	// User can send pull request if owns a forked repository.
-	if ctx.IsSigned && repo_model.HasForkedRepo(ctx, ctx.Doer.ID, ctx.Repo.Repository.ID) {
-		ctx.Repo.PullRequest.Allowed = true
-	}
 }
 
 func retrieveProjectsInternal(ctx *context.Context, repo *repo_model.Repository) (open, closed []*project_model.Project) {