Revisit send to kindle integration

[facundoolano]

I currently rely on stkclient to send articles to kindle devices. This is sketchy because it mimics a desktop app to get a private key and brittle because the key seems to expire within a couple of days, requiring another manual login. I'm thinking at this point it's probably better to just send the parsed webpages through email.

This would require:

• setting up an email account or service via configuration (in the production config).
• allowing the user to enter their kindle device email (a field in the user model that would replace the current kindle related table)
• whitelisting the feedi instance email in the user amazon account.