Session closing

[biozz]

We need an ability to «close» the session. Session closing means that:

• there is a dedicated button to close the session
• the same button can be used to re-open the session
• when the session is closed, the code editing is disabled

There are no restrictions on who can close the session. It is just an indication, that there is no activity expected in the document.

We could introduce a new field: state. It could be in one of «open» or «closed».

The button must be in the Ostrovok! color scheme.