Add ability to unlock scrolling in output view

[bpringe]

Currently the output view always scrolls to the bottom when new output is appended, even if the user has scrolled up. We should probably:

• Lock scrolling by default
• Unlock scrolling when the user scrolls up / when the Y scroll value is no longer the maximum.
• Show a button that is absolute positioned that the user can click to lock scrolling again
• Lock scrolling automatically when the Y scroll value is the maximum (nice to have? maybe add later?)
• Save whether scroll lock is enabled in state when it's changed and use that value to set scroll locking when the output view is reloaded