Implement Firefox account authentication

TODO:

• Request to register ffsend as OAuth relier for Firefox Accounts
• Assign/receive application secret to use for OAuth
• Implement OAuth authenticate procedure in ffsend
• Build some sub-commands to manage authentication

Interesting options to use:

• https://crates.io/crates/oauth2_noserver

Related links:

• https://mozilla.github.io/application-services/docs/accounts/welcome.html
• https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Firefox_Accounts/Introduction
• https://github.com/mozilla/fxa
• https://github.com/mozilla/application-services