Improved Restrict Access widget

We need to make the Restrict Access widget more intuitive. Shortcodes will no longer be necessary for most use cases.

### Notify customers

- [ ] Gmail Label: **Follow up/memberful-wp:50**.
- [ ] Follow up with [customer](https://basecamp.com/1727448/projects/1780718-memberful-beta/messages/12492909-custom-message).

## Phase 2

- [ ] Add buttons in the editor for purchase links