@fastlane-bot migration to GitHub Actions

### Introduction

Based on [the proposal](https://github.com/fastlane/fastlane/issues/15078), 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](https://github.com/actions/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](https://github.com/fastlane/fastlane/issues/15078#issuecomment-514307813)
- [x] Marking a PR as merged:
* [saying thank you for a contribution](https://github.com/fastlane/fastlane/pull/15042#issuecomment-512955088)
* 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](https://github.com/fastlane/fastlane/pull/15013#issuecomment-510622687)
* [adding a comment to the referenced issue that the PR has been released](https://github.com/fastlane/fastlane/issues/15145#issuecomment-526721562)
* 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.