Add undo button for `sync-pr-commit-title` and `clear-pr-merge-commit-message` #4606

Since these 2 features are "destructive", maybe we should add a button to allow the user to undo them in place.

<img width="704" alt="Screen Shot" src="https://user-images.githubusercontent.com/1402241/126858267-7bfce07f-f625-4b8b-ab00-ed4ec9bc3a4b.png">

How:

• both features should try to add that notice if they changed the content
• the original value of the title is stored in the value attribute and the textareas’s is in the textContent property
• the button should read these 2 original values, set them back, and disappear

The logic should probably be in sync-pr-commit-title.tsx:

• export function addNotice() {add notice if it doesn't exist}
• function onButtonClick() {removeButton && removesync-pr-commit-title’s checkbox}

Related:

• https://github.com/sindresorhus/refined-github/issues/3964

FYI: sync-pr-commit-title has 2 parts:

• sets the commit title to the PR title
• changes the PR title