drafts not being displayed when clicking between sources

[redshiftzero]

I just noticed this while trying to get STR for #734

STR

0. Apply server diff that will fail replies after a delay:

diff --git a/securedrop/journalist_app/api.py b/securedrop/journalist_app/api.py
index cf6d76585..8cd3e8a61 100644
--- a/securedrop/journalist_app/api.py
+++ b/securedrop/journalist_app/api.py
@@ -237,6 +238,15 @@ def make_blueprint(config):
             if 'reply' not in request.json:
                 abort(400, 'reply not found in request body')

+            import time
+            time.sleep(5)
+            abort(409, 'That UUID is already in use.')
+
             user = get_user_object(request)

1. Send test reply to a source
2. Click to another source
3. Click back to the initial source

Expected behavior

Draft is shown in whatever state it's in (pending, successful, failed)

Actual behavior

Draft is not displayed in conversation view

[redshiftzero]

seems like we are deleting the DraftReply objects in https://github.com/freedomofpress/securedrop-client/blob/master/securedrop_client/gui/widgets.py#L2500

[redshiftzero]

seems like we do this in case the draft was converted to a successful reply. however, we still have draft replies we do want to display in their pending or failed state in the conversation view