Rework on code review process

Summary | 概述:

1. We are no longer using the changelog directory, and will be using the title and description of the MR/Differential Revision as the changelog entry instead.
2. For an MR, this means all commits will be squashed.
3. The original check-changelog job is removed, and we will no longer have duplicated pipelines for branches and MRs.
4. For a differential revision, use arc land to land the changes.
5. When running arc diff, it will push to a staging area, which is a GitLab repo, and the pipeline will be triggered from Phorge. The link to the pipeline will be reported to the build target.

https://iron.lily-is.land/T1

Type | 类型: skip

Test Plan | 测试计划:

1. For MR:

   1. Create an MR based on this.
   2. Verify that check-changelog won't run.
   3. Verify that report-start and report-final won't run.
   4. Verify that all other jobs are still run.
   5. Verify that you can squash the commits when merging, and the commit message contains the title and full description of the MR. DO NOT merge yet.

2. For Differential Revision:

   1. Create a Differential Revision based on this (very important) (like arc diff HEAD^). It will ask you to push to staging area.
   2. Verify that report-start and report-final are both run for phabricator/diff/xxx.
   3. Verify that after report-start, you can see in Harbormaster that the build is in progress and the link.
   4. Verify that after report-final, you can see in Harbormaster that the build is finished.
   5. Update the diff to make the build fail, and you should see in Harbormaster that the build is failed.
   6. Verify that the pipelines are NOT run for phabricator/base/xxx tags.
   7. Verify that if you do not include a Type: specification in the Revision summary, it will add a comment asking you to do so.

.gitlab/merge_request_templates/Default.md

+Summary | 概述:
+<!--
+    (Please remove the comments when submitting the merge request.)
+    （在提交合并请求的时候请删掉这些注释。）
+    Describe in detail what the changes are.
+    详细描述改变了什么。
+-->
 
-# Checklist | 检查清单
-- [ ] Unit tests | 单元测试
-- [ ] CHANGELOG | 修改记录
+Type | 类型: <!-- add|remove|skip|security|fix -->
+
+Test Plan | 测试计划:
+<!--
+    Tell reviewers how to verify the intended behaviours.
+    告诉审核者怎么验证想要的表现。
+-->