Expose API

[robin-drexler]

First iteration

• retrieve printers
• init printing process (probably just use existing endpoint, CSRF protection would have to be disabled cough)

Second iteration

• allow overriding of color and ticket type from url parameters, as different issue trackers may have configured different types and colors
• id field should be able to repo names like Jimdo/JimFlow #333 as #numbers are ambiguous otherwise