Add forget history up to this point

[slatinsky]

This pull request:

• adds ability to "forget history up to this point" - user sets it by clicking the buttons

• "memory marker" indicates, which messages are included in the chat (below the marker) and which are not (above the marker)

• summarize button now just moves "memory marker" instead of deleting the entire message history

Implementation details:

• adds sendFromIndex to conversation object

• migrates old conversations to the new format (with added sendFromIndex field)

• unrelated small fix - adds minimum height to system message in sidebar (that was too small if threads don't fit on the screen)

Fixes issues:
#25 - if the user uses "forget history up to this point" feature, many tokens are not used up
#6 - another way to handle this closed issue

Bugs:
#12 - the same issue - refreshing anything in $conversations repaints the entire conversation - and scrolls down