Issue #343: Browser back button broken in source viewer (conservancy/kallithea)

[robagar at bitbucket.org]

New issue 343: Browser back  button broken in source viewer
https://bitbucket.org/conservancy/kallithea/issues/343/browser-back-button-broken-in-source

Rob Agar:

The Chrome browser back button has no effect when viewing a file source.

This error appears in the console

```
ExampleDevice.yaml:671 Uncaught ReferenceError: State is not defined
at load_state (ExampleDevice.yaml:671)
at ExampleDevice.yaml:793
```

I thought the fix would be a simple typo  fix “State.url” → “state.url” in _templates/files/files.html_

```
104             asynchtml(State.url, $files_data, function() {
105                   post_load_state(state);
106                   var expire_on = new Date().getTime() + CACHE_EXPIRE;
107                   CACHE[cache_key] = [expire_on, $files_data.html()];
108             });
```

but this change gets rid of the console error, but does not actually make the back button work