[UX] Saving a forked livebook should default to the same folder

[derheap]

I often fork livebooks (mostly for reusing the connection to an elixir project).

But while saving the fork, I have to wade through all my directory structures again, to save the forked livebook beside the original one.

It would be a lot easier, if the save dialog, would start then not on the root directory, but in the directory of the livebook I just cloned.

[josevalim]

Sounds good to me. Can you please open up an issue and potentially submit a PR? I think it should be easily doable because I believe we already keep around the information about which Livebook we forked from.

[derheap]

Part one done.

I have never looked into the livebook source before, so the PR could take a while.