Introduces ZenodoClient and fixes API usage for submitting new concept versions

[Tomaz-Vieira]

This PR introduces the ZenodoClient class, as well as some serializable data types that are useful for communicating with Zenodo in a type-safe way.

• A ZenodoClient class that keeps a requests.Session to speed up communication
• All Zenodo endpoints that are relevant for backing up models are encoded as methods in the client
• (De)serializable classes map the data expected and produced by the Zenodo API

TODO:

There is still at least one corner-case where our backups could break in a way that is not automatically recoverable:

1 - Create a new concept version
2 - Add some files and metadata to it
3 - Crash before publishing (e.g. network errors or the like)
4 - Lose the latest Record ID

Now no new versions can be created, and the staging Record can't be deleted, for it does appear in queries to the GET /api/deposit/depositions endpoint. The only way to recover from this is for a human to go to zenodo.com and manually delete the staging record.

I'm waiting on a response from the Zenodo team in case there is a way to work around this.