Add deadline to todo

[theDisco]

• User should be able to add a deadline as a date in the future to a todo
• User should not be able to add a deadline, if the date is in the past

[codeliner]

👍

One addition: User must be the owner of the todo

FYI: I'm currently planing oauth2 integration and also moving user handling into its own context. This change should not effect your task that much.

When we have the deadline feature in place we can think about how to monitor the deadlines.

[theDisco]

Merged into master.

[codeliner]

@theDisco small correction: PR is merged into develop. As soon as #40 and #39 are also merged into develop, we will merge all changes into master and release a new version.

[theDisco]

Old habits :D