@fastlane-bot migration to GitHub Actions

[mollyIV]

Introduction

Based on the proposal, we would like to migrate to GitHub Actions. This will bring a bunch of improvements:

• well tested and small, single job focused code (an action)
• bot's reactiveness improvements (by using web hooks)
• easier contribution
• no need for running a dedicated server for a bot
• there are plenty of actions ready to be used in production, i.e. stale
• switching the mindset from fastlane bot code to reusable and useful for others
• bot's icon and messages improvements
• documentation

GitHub Actions to be implemented and activated

We will need to implement the following actions:

• [x] Asking about fastlane env information
• [x] Marking a PR as merged:
  • saying thank you for a contribution
  • adding status: included-in-next-release label to the PR
  • removing status: needs-attention label from the PR
• [x] Marking a PR as released:
  • informing about including the changes from a PR in a release
  • adding a comment to the referenced issue that the PR has been released
  • adding status: released label to the PR
  • removing status: included-in-next-release label from from the PR
• [x] Locking closed issues and pull requests that have not had recent interaction
• [ ] Sending a message to Slack channel about PRs that have status: needs-attention label

Anyone is more than welcome to help 🙌 Please open a new issue for an action you'd like to implement and link it to this very issue.