[FEATURE]: Add a way to bookmark sessions to make finding them again later easier.

[ariane-emory]

Sometimes, I implement some feature/fix in one session, and then I spend a few days testing it and feeling it out or working on different features/fixes.

Then, later on, I wanna go back to that original session - where everything needed to understand the feature/fix is already in the context so that I don't need to explain everything to the model again - to ask for some revision.

But actually finding that original session in the session_list menu again can be tricky because I've engaged in a gajillion other OC in the intervening time, and that original session is now buried pages and pages down in the session_list menu.

So: let's add a new feature that allows the user to bookmark sessions to make them easier to locate again later. Bookedmarked sessions should be grouped separately near the top of the session list (similarly to the way that favorited models are grouped separately near the top in the modal window used by the /models command).

Let's use ctrl+d as the default key to bookmark or unbookmark sessions, since this is the key that is often usualy used by web browsers to bookmark web pages.