Slow-start-rewatch

- Add `ruamel.yaml` to dependencies (missing when installed on Windows)

- Refactor the `Scheduler` to improve the exception handling when loading the scheduled post
- Move the `EmptySchedule` checking from the `Timer` to the `Scheduler`
- Add the `RedditHelper` class - providing the Reddit's API methods unsupported by `PRAW`
- Add the `PostImage` class - representing an image in a Reddit post
- Add the `TextPostConverter` class - converting from Markdown to Reddit Rich Text JSON
- Update the `Post` to include the attributes needed for submitting a post with thumbnail
- Implement loading of posts with thumbnails to the `Scheduler`
- Implement post submission with thumbnails to the `RedditCutifier`
- Implement post updating to the `RedditCutifier` for restoring the original Markdown of the posts with thumbnail

- Implement the post submission to the `RedditCutifier`
- Add the post submission via the `RedditCutifier` to the `App`
- Add the `ConfigStorage` class - storing the refresh token
- Implement the storing of the refresh token to the `Config`
- Add the authorization with the refresh token to the `OAuthHelper`

- Add `Jinja` templates with cute GIFs - used for the OAuth callback
- Add `flask` to dependencies
- Add the `http_server` module - handling the OAuth callback
- Add `praw` to dependencies
- Add the `OAuthHelper` class - providing the methods for the Reddit authorization
- Add the `RedditCutifier` class - making Reddit a cuter place
- Add Reddit authorization via the `RedditCutifier` to the `App`

- Add class `Timer` - responsible for waiting until the scheduled time
- Add the `Timer` to the `App`
- Add rendering of the remaining time during the countdown

- Add class `Post` - representing the Reddit post
- Add class `Scheduler` - managing data about scheduled posts
- Add a creation of a sample scheduled post
- Add the `Scheduler` to the `App`