UI for encryption before upload

[lidel]

Part of UX improvements for sharing local files (#342)

Idea extract from https://github.com/ipfs-shipyard/pm-ipfs-gui/issues/34#issuecomment-380691178 and https://github.com/ipfs-shipyard/ipfs-companion/issues/342

Provide easy way to share secrets

• encryption and decryption happen client-side (JS)
  • use well known, audited library
• an option to encrypt content before upload via AES
  • uploader can choose enter a secret (password) for encryption or it will be generated automatically
    • ideas: it can be a "password" in proquint form, or a "key" in hash part of the URL
  • produce additional static landing page that instead of details about uploaded data prompts receipent for secret required for decryption
  • after decryption, show regular landing page (https://github.com/ipfs-shipyard/pm-ipfs-gui/issues/34#issuecomment-373926710)