Read-only mode

**Is your feature request related to a problem? Please describe.**
I find `lazygit` both extremely cool, and terrifying at the same time. The fact that I can do so much with one or two keystrokes is amazing, but means that I'm not comfortable leaving a `lazygit` shell open in a tmux session as a form of HUD for fear that I'll accidentally press a button and mess something up

**Describe the solution you'd like**
It would be great if there were a way to activate (possibly by default) read-only mode, so that none of the actions that change the repo work. This way I could use `lazygit` just for its nice commit-browsing functionality and leave it open without fear of messing something up accidentally. Then there would be some keystroke / action I could take to switch to normal/write mode when I need to do something

**Describe alternatives you've considered**
Alternately, we could just have a command to disable keybindings altogether until another command was issued, but then you lose the ability to do navigation commands without potentially missing a key and messing something up

**Additional context**
Add any other context or screenshots about the feature request here.