no longer try to show conversation for deleted src

[sssoleileraaa]

Description

Fixes #302
Fixes #297
Fixes #195

Summary of Changes

• on_source_changed now checks if the current source exists before trying to set the conversation
• on_source_changed clears the conversation from the conversation view if the source does not exist
• Removed calls to self.controller.session.refresh since it isn't necessary (might have been a fix for some bug that no longer exists)
• We now show the number of replies being deleted in the popup that checks if the user wants to delete a source. Before we showed only counts for messages and files.

Test

1. Log into the client
2. Delete the first source by selecting 'x' next to the source in the source list and see conversation view cleared (app no longer crashes) after source and conversation are deleted
3. Delete the second source using the source menu in the far upper-right corner and see conversation view cleared (app no longer crashes) after source and conversation are deleted
4. See how conversation view is empty when the last source has been deleted

[redshiftzero]

diff looks good now, I followed the test plan and all three bugs resolved as advertised! 🥇