The existing code downloaded any attachments to the user's "Downloads"
folder. If the user is logged in with several accounts these downloads
will be mixed up together.
Fix this by adding a new preference that allows the user to specify the
downloads should be placed in a sub-folder per account, named after the
account.
To do this:
- Add an interface for enums that can be used as preferences, with
properties for the string resource to display and the value to store.
- Add `EnumListPreference`, a `ListPreference` that allows the user to
choose between different enum values.
- Add a `DownloadLocation` enum and preference key so the user can
choose the location.
- Add a `core.domain` module, with a use case for downloading URLs that
respect's the user's download preference. Use this use-case everywhere
that files are currently downloaded.
Fixes#938
