Introduce `gaiacli keys export` / `gaiacli keys import`

[cwgoes]

We definitely need these prelaunch. They can come with extensive warnings.

Ideally we should do this in a standard format; the ASCII-armored one currently in the keybase implementation should be fine for now.

[cwgoes]

Brought up by @alessio - cc @ebuchman was this controversial?

[zmanian]

Our view has that people need to store their words and that's our portability solution.

I disagree that this is prelaunch cause no one will be able to transfer funds at launch.

[rigelrozanski]

second zmanian, exporting the seed phrase should be sufficient

[cwgoes]

second zmanian, exporting the seed phrase should be sufficient

That would be fine if it were possible but it isn't because we don't store the seed phrase. I think we want some export method that can happen after you first create the key, otherwise users who fail to write down their seed phrase - but still have the gaiacli DB and their password - will find it very difficult to recover their keys.

It's a non-breaking change - so OK on post-launch - I expect we'll get requests for this pretty quickly.

[jackzampolin]

Sounds like we still don't have gaiacli keys export

[alexanderbez]

Correct, we do not have this atm. As I see it, we can:

1. Expect users to write down/safely store their seeds (as they should as it is up to them) and this is generally a good practice users should be doing anyway. If they lose a seed -- they're screwed.
2. Potentially store the seed and allow that to be exported. I'm less worried about this now since we updated the permissions on the keys DB, so only a system user with the correct privileges would be able to execute such a command.