Allow bookmarking messages from closed posts

[shtlrs]

Currently, Sir Lancebot cannot bookmark messages that are part of closed posts and according to @HassanAbouelela, this is due to the nature of discord.py (discussion)

Chris suggested to use a PartialMessage, and if the content is not available, we can just use its jump_url

Would you like to implement this yourself?

• [x] I'd like to implement this feature myself
• [ ] Anyone can implement this feature