GitHub workflows are out of date

[mcmire]

We need to align this repo with the module template (or some other project that not only publishes to NPM but also deploys a site).

Also see #144.

Acceptance Criteria

• The set of GitHub workflows in this repo matches the set defined in the module template.
• We are using the same versions of actions as is used in the module template (create-release-pr, npm-publish, etc.)
• Merging a release PR automatically deploys to https://metamask.github.io/test-dapp/ and also https://metamask.github.io/test-dapp/<version>. (The former automatically redirects to the latter.)
• Pushing a commit to a release PR automatically deploys to https://metamask.github.io/test-dapp/rc-<version>.