ConversationView: add current user to conversations, according to Mastodon behaviour, fix last_status.account being not filled

Mastodon doesn't delete current user from conversations.

This fixes #2217 (closed) but "account" field of "last_status" is still empty. I don't know what causes it.

References:
0. https://github.com/tootsuite/mastodon/blob/master/app/models/account_conversation.rb