Custom Installer Builder should not let users proceed with only private key(s) downloaded

[choksi81]

After building a custom installer that has server-generated keys inside (step 1 in the Custom Installer Builder), the clearinghouse instructs users: "You must download an archive of the private keys that were generated for new users." Only when done, buttons to proceed further are unlocked.

One of the unlocked buttonsis labeled "Download public keys", the note to its side reads "It is strongly recommended that you download an archive of the public keys of all users." Download is not enforced here.

If the user only went with the private key, they will have problems later when using seash, as it requires a key ''pair'' to work. Now technically we could regenerate the public key from the private key, but there is no ready-to-use way for this in our libraries.

We should require that the user downloads the public key(s) for server-generated users too. A fix for #1333 would properly fix this issue here too.

Credits go to Rob Chiarelli for reporting this issue!