Associate projects with user IDs in the model

[create-issue-branch]

Closes #16

• Moves remix endpoint to it's own controller to be more RESTful
• Create operation to handle business logic
• Ensure user_id is stored against remixed project

Sending the User ID in the request body is not ideal and will be changed to require a user token to be included in the request header. The API can then check this via Hydra to authorize the user and obtain user details.
This would prevent anybody from being able to create content under another users details (as the API is public)

[raspberrypigithubbot]

• CircleCI build #44
• Test coverage: 61.53%

[raspberrypigithubbot]

• CircleCI build #46
• Test coverage: 61.53%

[raspberrypigithubbot]

• CircleCI build #48
• Test coverage: 78.26%

[raspberrypigithubbot]

• CircleCI build #50
• Test coverage: 78.26%

[raspberrypigithubbot]

• CircleCI build #52
• Test coverage: 77.47%

[raspberrypigithubbot]

• CircleCI build #54
• Test coverage: 77.47%

[raspberrypigithubbot]

• CircleCI build #56
• Test coverage: 77.47%

[raspberrypigithubbot]

• CircleCI build #58
• Test coverage: 77.47%

[raspberrypigithubbot]

• CircleCI build #60
• Test coverage: 78.37%

[raspberrypigithubbot]

• CircleCI build #62
• Test coverage: 78.18%

[raspberrypigithubbot]

• CircleCI build #64
• Test coverage: 79.09%

[loiswells97]

Looks good to me 👍