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/api/v1/repo/branch.go

@@ -155,7 +155,7 @@ func DeleteBranch(ctx *context.APIContext) {
 		case git.IsErrBranchNotExist(err):
 			ctx.APIErrorNotFound(err)
 		case errors.Is(err, repo_service.ErrBranchIsDefault):
-			ctx.APIError(http.StatusForbidden, errors.New("can not delete default branch"))
+			ctx.APIError(http.StatusForbidden, errors.New("can not delete default or pull request target branch"))
 		case errors.Is(err, git_model.ErrBranchIsProtected):
 			ctx.APIError(http.StatusForbidden, errors.New("branch protected"))
 		default: