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>

templates/repo/settings/options.tmpl

@@ -287,6 +287,7 @@
 			</div>
 		{{end}}
 
+		{{/* FIXME: need to split the "Advance Settings" by units, there are too many options here */}}
 		<h4 class="ui top attached header">
 			{{ctx.Locale.Tr "repo.settings.advanced_settings"}}
 		</h4>
@@ -594,6 +595,20 @@
 								</div>
 							</div>
 						</div>
+						<div class="field">
+							<label>{{ctx.Locale.Tr "repo.settings.pulls.default_target_branch"}}</label>
+							<div class="ui search selection dropdown">
+								<input type="hidden" name="default_target_branch" value="{{$prUnit.PullRequestsConfig.DefaultTargetBranch}}">
+								<div class="default text"></div>
+								{{svg "octicon-triangle-down" 14 "dropdown icon"}}
+								<div class="menu">
+									<div class="item" data-value="">{{ctx.Locale.Tr "repo.settings.pulls.default_target_branch_default" $.Repository.DefaultBranch}}</div>
+									{{range $branchName := $.Branches}}
+										<div class="item" data-value="{{$branchName}}">{{$branchName}}</div>
+									{{end}}
+								</div>
+							</div>
+						</div>
 						<div class="field">
 							<div class="ui checkbox">
 								<input name="default_allow_maintainer_edit" type="checkbox" {{if or (not $pullRequestEnabled) ($prUnit.PullRequestsConfig.DefaultAllowMaintainerEdit)}}checked{{end}}>