Generate certificate (2nd try)

[mphasize]

Story: "As an admin I want to generate the certificate PDF for a participant and see a preview image."

• Proof of concept with a demo layout
• Generate PDF when importing
• Generate preview image on demand (first load)
• Manage persistent storage in dokku
• Add managing/uploading templates with layout config?
• Add full color support to template layout configuration
• Add font italic option
• Edit Batch settings (startDate, endDate, ...)
• Action to update all certificates rendered from a template when the layout was changed (based on updatedAt timestamp)
• Action to update all certificates when the batch settings were changed (based on updatedAt timestamp)
• Throw error when importing participants without an email
• Add/delete batches
• Sort batches by date in descending order
• Add managing/uploading fonts?
• Manually set up new ENV vars in the production app on dokku
• Updated action for cloning the review app database (unlink cloned database from production app)

Gotchas: Font usage is hard-coded in the layout settings.

[mphasize]

After merging #7 I rebased this PR and therefore had to force push the commits again.