migrate sqlite db users table to a per-maildir "password" file

[hpk42]

• set/read passwords from a "password" file in maildir
• set/read passwords for echobot from a special password file created by the echobot
• track last_login for a user as the "mtime" of the password file
• migration script for sqlite database to files (moving the old passdb.sqlite to passdb.sqlite.old)
• new "User" object for manipulating passwords and last-login timestamp for a user with tests

supersedes #354 (this PR is based on it, but it's better to merge it all in one go, to not have partial db->file migrations)

[missytake]

Can you maybe change the HEAD this PR is to be merged into to #354? It would be easier to review then - first we could review #354, then this one, then merge this, and then (possibly without further review) #354. Breaks it up into smaller chunks

[hpk42]

Can you maybe change the HEAD this PR is to be merged into to #354? It would be easier to review then - first we could review #354, then this one, then merge this, and then (possibly without further review) #354. Breaks it up into smaller chunks

I re-targeted the branch to merge with hpk/last-login which was already reviewed.
Note btw, that even though there is a large "migrate_from_db_to_maildir" script with a large test,
this PR cuts out lines because all the sqlite-handling is gone.