feat: add button to quickly clear merge message (#21548)

This PR adds a button to allow quickly clearing the merge message of a
PR. The button will remove everything but the git trailers.

I found myself often pruning the commit message before merging,
especially for PRs generated by renovate - renovate puts a very long and
detailed comment with the full changelog in each PR it opens. This
clutters the commit message. However, I want to explicitly preserve the
git commit trailers. Doing this manually works, but having a button is a
lot easier.

Screenshot:

![image](https://user-images.githubusercontent.com/13721712/197337525-d456d0f8-1f7c-43a9-815d-ca93b1e7a90a.png)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

templates/repo/issue/view_content/pull.tmpl

@@ -351,6 +351,8 @@
 									'textAutoMergeButtonWhenSucceed': {{$.locale.Tr "repo.pulls.auto_merge_button_when_succeed"}},
 									'textAutoMergeWhenSucceed': {{$.locale.Tr "repo.pulls.auto_merge_when_succeed"}},
 									'textAutoMergeCancelSchedule': {{$.locale.Tr "repo.pulls.auto_merge_cancel_schedule"}},
+									'textClearMergeMessage': {{$.locale.Tr "repo.pulls.clear_merge_message"}},
+									'textClearMergeMessageHint': {{$.locale.Tr "repo.pulls.clear_merge_message_hint"}},
 
 									'canMergeNow': {{$canMergeNow}},
 									'allOverridableChecksOk': {{not $notAllOverridableChecksOk}},
@@ -360,6 +362,7 @@
 									'defaultMergeStyle': {{.MergeStyle}},
 									'defaultDeleteBranchAfterMerge': {{$prUnit.PullRequestsConfig.DefaultDeleteBranchAfterMerge}},
 									'mergeMessageFieldPlaceHolder': {{$.locale.Tr "repo.editor.commit_message_desc"}},
+									'defaultMergeMessage': defaultMergeMessage,
 
 									'hasPendingPullRequestMerge': {{.HasPendingPullRequestMerge}},
 									'hasPendingPullRequestMergeTip': {{$hasPendingPullRequestMergeTip}},