Allow importing seeds into lair

[matthme]

• [x] read data from json file or user input
• [x] encrypt seed using x_salsa20_poly1305_encrypt
• [x] use lair's import_seed to import the seed into lair with an appropriate tag (logic to be decided, probably a prefix for imported seeds)
• [x] derive specific keypairs based on the imported seed if necessary
• [x] add UX for all of the above