nitpicking : allow edit in read only mode, whilst disabling saving

This would help people who wanna make a quick edit to a file in a repo and share it with someone, instead of having to create a new file, copy the contents into it, etc.

Also, a new file can't be saved using just github1s, and using an already saved file would unintentionally give the user the required syntax highlighting without them having to choose the language mode for each file.