Allow a user to disable an email account by setting
`accounts.email.accounts.<name>.enable = false`. This is useful if
someone wants to configure email accounts globally but only use them in
certain circumstances.
Everywhere email account configuration is used, check if the account is
enabled before checking any attributes of the account.
The git-send-email [0] script uses StartTLS if `smtpEncryption` is set
to `tls`, which can break services that don't support StartTLS.
[0]: bd42bbe1a4/git-send-email.perl (L1533)
PR #1395
