repl: add option to display line numbers

[roipoussiere]

fix #575

I had to remove this in website/src/styles/index.css:

.cm-gutters {
  display: none !important;
}

There was a particular reason to hide cm gutters in css?

[felixroos]

I think I initially hid them because they were a bit mich for the mini repl and also a bit bulky in the main repl.
It might be best if the gutters can be toggled in the settings. I can also do that if you want

[felixroos]

ah ok sorry I should have read the code before commenting.. now I see it is actually togglable, great!

[felixroos]

is this ready to merge?

[roipoussiere]

yep!