This allows uploading designs through GraphQL.

The mutation requires these arguments:

- `fullPath`: The path in which we can find the issue for creating
  designs
- `iid`: The iid of the issue in which to upload designs
- `files`: An array of files. We expect these to be in the format
  used by apollo-upload-client[0] which uses this spec[1] for
  multipart fileupload.

The middleware used for handling the multipart upload an translating
the variables is apollo_upload_server[2]

When a file is uploaded, the basename is used for creating a design,
when a design for the specified issue did not exist for the filename,
a new one is created. Otherwise the existing one is used.

When uploading a new file, a new version is created an linked to the
design being updated or created.

The files are stored in a repository that lives next to the project
repository with the path `@hashed/[hash]/[to]/[repo].design.git. The
files are not yet stored in LFS, but they should be.

0: https://github.com/jaydenseric/apollo-upload-client
1: https://github.com/jaydenseric/graphql-multipart-request-spec
2: https://github.com/jetruby/apollo_upload_server-ruby