Install Thunderbird language packs through enterprise policies so Home
Manager can use Mozilla's release-hosted XPIs without vendoring or
hashing them.
This also exposes programs.thunderbird.policies for configuring
supported Thunderbird enterprise policies directly and wraps the
configured Thunderbird package with merged extraPolicies.
Generate the Thunderbird identity and outgoing server preferences
needed for explicit accounts.email.accounts.<name>.ews accounts,
and add an outlook.office365.com-ews flavor preset for Office365
Exchange accounts.
Closes#8011
Add a new Thunderbird module that uses the configuration in
`accounts.email.accounts` to setup SMTP and IMAP accounts.
Multiple profiles are not supported at this point.
