/files/:file_id/convert endpoint should make use of job model

[sergiofenoll]

Note to self: link KAS ticket here if ever created

The file conversion can potentially take a long time because:

1. We're dealing with files, which can potentially be huge
2. We're making an outbound network request

If a frontend initiates the conversion process, the call itself might time out because it takes too long.
We should work with jobs in this service so that the process can be tracked and polled by the frontend.