Only allow one language per file?

[thecodingwizard]

Instead of having three tabs, we can also just only allow one language per file. It seems like this would simplify a bit of code + simplify the UI, and I can't really think of that many use cases where someone would want multiple languages in one file?

[thecodingwizard]

migration plan:

• add a changelog to the homescreen
• add "version" property to files
• for files with no version property, alert the user that the file is outdated and they should copy-paste into a new file, and then open it in an old version of the IDE

[thecodingwizard]

actually, it's easier implementation-wise if we just don't show any of the old files (and link to an old version of the IDE that does show old files)

[thecodingwizard]

This will be implemented in #94