Use the api to fetch the admin public key for users to send the decryption key securely

[Tbaut]

To report a file that may not comply with our privacy policies, users can send to an admin the path to the file that is concerned, along with the decryption key. For the first iteration of this feature, users need to send this info per email, and because this means of transport is insecure, the file decryption key should be first encrypted with an admin public key.

This admin public key is currently hardcoded in the UI, and it should rather be fetched from the api so that it can be rotated without any UI change.