Support multiple files in the synchronized editor

[MrSerth]

Currently, the delta is only applied to the first editor session. This will cause issues with exercises having multiple files, I assume.

The problematic line is:

codeocean/app/assets/javascripts/editor/editor.js.erb

Line 1027 in 89afb59

this.editors[0].session.doc.applyDeltas([delta]);

There, every change is inserted into the first editor (identified by this.editors[0]).

In order to support multiple files, we should probably send the active file ID together with the delta and then insert the change in the respective editor.