Implement opportunistic encryption in composer

When enabled KMail will automatically look for public keys for
each recipient and if it finds one, it will automatically enable
encryption. The key lookup includes fetching public keys from
a key server. A small icon will also appear next to each recipient
with more details regarding the trustiness of the key and will
display dialog with more details about the key being used for
encryption when clicked.

If we fail to obtain a key for at least one recipient, the email
won't be sent encrypted unless user explicitly toggles the
encryption on.

The feature is only enabled when current identity has "Automatically
encrypt messages when possible" enabled.

Differential Review: https://phabricator.kde.org/D1639