Show placeholder content in conversation view when client is empty or no source is selected

[sssoleileraaa]

Description

Instead of displaying a blank conversation view, display a placeholder that @ninavizz will provide for the cases where we don't know the user's last-seen source, a user is offline, and there is an empty source list. There will likely be a separate placeholder for empty source list.

Acceptance criteria

Application start: offline mode

1. application starts
2. user logs in in offline mode, sees placeholder that @ninavizz will provide

Application start: A's first session or A's last-seen source was deleted by another user

1. application starts
2. user logs in as A, sees placeholder that @ninavizz will provide

There are no sources in the list

1. application starts without sources in the list or user deletes the last source in the list
2. user sees placeholder that @ninavizz will provide

[eloquence]

The reviewed and approved placeholder designs can be found here:
freedomofpress/securedrop-ux#54 (comment)