Refactor routing to remove phrases part of URL

[ArayB]

Remove the phrases section of the api URLs so instead of:

/api/projects/phrases/project-id-phrase/remix

We have:
/api/projects/project-id-phrase/remix

See https://github.com/RaspberryPiFoundation/digital-editor/issues/11

[create-issue-branch]

Branch issues/25-Refactor_routing_to_remove_phrases_part_of_URL created!