Improve message visibility for added/removed users

[dtdesign]

The current implementation is somewhat weird, or at least a bit weird, when it comes to users being added to existing conversations. These users will see all messages of this conversation, regardless if this is the intended behavior or not. On top of that, the conversation starter can remove participants at any time, that not only locks people out of the conversation, but also virtually deletes it for them.

• Add two new fields to wcf1_conversation_to_user that track the time at which a user joined/left a conversation, the special value 0 equals "unrestricted".
  • joinedAt to show only messages that were posted exactly then and in the future.
  • leftAt is the opposite of joinedAt.
• Removing a user from a conversation sets leftAt, but does no longer change hideConversation.
• Add additional users to a conversation should offer the option to grant them full access (joinedAt = 0), or to restrict visibility of previous messages (joinedAt = TIME_NOW).
• Checks for hideConversation = 2 (STATE_LEFT) need to be adjusted to check for leftAt too.
• Add wcf1_conversation_to_user.lastMessageID, that is set whenever a user is removed from a conversation, making it faster to adjust the "last reply"-column.

See https://community.woltlab.com/thread/257471-konversation-teilnehmer-entfernen-verhalten-so-gewollt/
See https://community.woltlab.com/thread/258903-kein-verlauf-f%C3%BCr-neue-konversations-teilnehmer/