Add linkpay

[DinSmol]

close #2210
Replaced banklinks with linkpay (everypay).
Added linkpay to email and pdf.
Added qr to pdf.

[viezly]

This pull request is split into 5 parts for easier review.
👀 Review pull request on Viezly

Changed files are located in these folders:

• /
• app/controllers/registrar
• app/jobs
• app/mailers
• app/models
• app/views
• config
• test

[vohmar]

Few issues:

• replace the payment button in registrar with a button called pay - it shows currently card payment icon that can be confusing
• the payment windows should be openened in new tab or browser window so user can close it after the payment process is finished

now the important one

• even though the payment was successful the invoice was not marked as paid