Support pinned events

[Nightchicken]

Added commands for pinning and unpinning events
:pin and :unpin
Window screen for pinned messages under :pinned which contains events within room

pressing enter on an even within the window jumps cursor to that event.
There are currently limits for how far back in history it can grab, events that are more than ~1500 messages will fail to load from scrollback, or if a time limit of 2 minutes occurs.

#626

[Nightchicken]

Moving stuff to fit with suggestion of having pinned messages be stored in scrollback with hidden tags.

[ulyssa]

I think this is in a good state for merging as a starting point, but ideally we will want to figure out a way to get the messages to render using the show/show_with_preview methods. That's going to take some work in modalkit-ratatui first though, so that the ListItem can render images, and so that the lifetimes it uses on ListItem::show allow the return value to borrow from the store.