Implementation of PayPal (Mock Up) as actual payment option

[LukasMeister]

User story

1. As a user
2. I want to be able to connect my PayPal account with the app
3. So that i can make purchases

-- correlates with issue #55 "select payment option" --

Acceptance criteria

• Has Paypal connection set up in the back end
• Once PayPal button is clicked the user is forwarded to the PayPal UI
• Backend sends arbitrary request to PayPal api (we believe that is possible)
• Once a user entered his credentials and logged into Paypal it is saved in his profile as established payment method
• Once that process is finished, the user is directed back to the select payment screen and can proceed with saving etc. (refer to issue #55 on how to proceed)

Definition of done

• Added only after week 5
• The same for all features