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/branch.go

@@ -41,6 +41,7 @@ func Branches(ctx *context.Context) {
 	ctx.Data["AllowsPulls"] = ctx.Repo.Repository.AllowsPulls(ctx)
 	ctx.Data["IsWriter"] = ctx.Repo.CanWrite(unit.TypeCode)
 	ctx.Data["IsMirror"] = ctx.Repo.Repository.IsMirror
+	// TODO: Can be replaced by ctx.Repo.PullRequestCtx.CanCreateNewPull()
 	ctx.Data["CanPull"] = ctx.Repo.CanWrite(unit.TypeCode) ||
 		(ctx.IsSigned && repo_model.HasForkedRepo(ctx, ctx.Doer.ID, ctx.Repo.Repository.ID))
 	ctx.Data["PageIsViewCode"] = true