Improve release workflow

[stevegrunwell]

This plugin is already using the 10up/action-wordpress-plugin-deploy GitHub Action for deploying new tags to the WordPress.org plugin repo, but there are still a number of manual steps.

Ideally, the process would look like this:

1. Create a new release/vX.X.X branch off of develop, bumping versions, updating changelogs, et al
2. Open a new PR against master with the following format:
   • Title: Version X.X.X
   • Body: (The contents that should eventually end up in the release)
   • Label: release
3. Upon merge, create a new release using the title and body of the PR, along with the v.X.X.X portion of the PR's branch name
4. As the tag is created, let the 10up workflow deploy the new version to the WordPress.org plugin repo
5. Automatically merge master into develop, ensuring the branches are even (or, at the very least, that develop isn't behind)

Some actions that may be helpful (opting to use official GitHub actions when possible):

• https://github.com/actions/labeler
• https://github.com/actions/github-script
• https://github.com/actions/create-release