make the option of random-key encryption available through the wui and cli

[zooko]

For #293 I added a parameter to upload functions to indicate whether we wanted content-hash-key or random-key encryption. Now I want to add a way for users to control this parameter through the wui and the cli.

For the wui, I want a checkbox next to the "mutable?" checkbox, which says "content hash key?". Of course, the question of content-hash-key or random-key applies only to immutable files -- for mutable files the key is always deterministically derived from the public key and the encrypted salt.

I don't know how to make this wui nice -- either have a checkbox for content-hash-key which dynamically appears whenever you select immutable-file, or else have a radio-button-selector offering mutable file, immutable file content hash key, or immutable file random key.

I hope that a wui wizard will address this issue, but in the meantime, I'm just going to add a separate checkbox with a label like "for immutable files: content-hash-key Y/N".

As for the cli, we can add a --content-hash-key=yes/no option, and raise a UsageError? if someone passes both --mutable and --content-hash-key.

[terrell]

if they're mutually exclusive - use a radio button. no reason to have more than one option/flag to keep track of.

[warner]

I've started working on this a bit, in conjunction with the "backupdb" and the tahoe cp command (since the CLI could do the CHK computation itself, and if it could pass the results to the node via webapi and key=, then we wouldn't incur a second hashing pass, and when the webapi becomes capable of doing streaming upload with pre-computed keys, we wouldn't incur a second IO-to-temp-disk pass either).

[zooko]

Nowadays I'm not sure that I actually want users to have the option of random-key encryption, as such, but I do want them to have the option of streaming upload (#320), which would require random-key encryption. Merging this ticket into that one.

[davidsarah]

See also #1288 (support streaming uploads in uploader).