Add alerting

[grittygrease]

If someone attempts to decrypt a file for which does not have the appropriate delegations, it currently returns with an error. This workflow can be improved with an alerting infrastructure.

On first decryption attempt: create a decryption request, which contains references to each of the users in the owner set of the file to be decrypted. If there are not enough keys delegated, select the users who can enable the decryption and send them a notification containing the following:

• which user is requesting decryption
• which labels are tied to the file
• a link to the delegation page with default values filled in (requesting user, labels, number of decryptions: 1)

Once enough owners have delegated the key for the file, the original requestor is given a notification that the file can be decrypted. Alternatively, the original request can be made to block until the file can be decrypted.

[grittygrease]

@ejcx Can you update this description to more accurately describe your new plan to use "orders"?