chore: remove unused components Manage, Navigation & Find Reviewers
A few things here:
- Removes Manage and Navigation components, unused and old.
- Removes Find Reviewers (pubsweet-component-find-reviewers) component, unused and old (we could update and revive it, if the need arises)
- Removes node-sass and style-loader from the styleguide (as we're now exclusively using styled-components)
- Enables linting for styled-components
- Fixes newly discovered linting errors