Bookmark feature for cryptogull

[visceralfield]

Ereshkigal has a bookmark feature that lets you use a reaction to save a message to your DMs. When you react to something with the 🔖 emoji, you get a DM that looks like this:

The Ereshkigal Prime project aims to make Erishkigal a bot for only administrator-permissions-required features, so Ereshkigal will no longer have this feature. We could port the bookmark feature from the current Ereshkigal (LHBot) to Cryptogull, since it doesn't require any admin permissions.