Check for premium before deleting, in case premium access is stale - #86

never stored locally and thus never deletable, leaving conversations that
couldn't be cleaned up.

- chat-indexer: save all real messages (isMessageView) instead of filtering
  on text; skip only placeholders
- getConversationMessageCount: count only real messageViews
- delete-messages: run empty-conversation cleanup from the DB even with no
  messages to delete; schedule the job whenever deleteChats is on
- FinishedModal: report conversations left
- update tests/fixtures accordingly