Issue 587 favorite removal errors

This fixes #587.

However, anyone who experienced the problems might experience residual disruption from them. If you do notice continuing problems removing favorites on trying a fixed version of the app, here are instructions that may clear up the effects. These measures will result in loss of any favorites that you have marked.

* On iOS, I think that just uninstalling the app before installing the new version will do it.
* On android in addition to removing the app before reinstalling, you need to use adb (the Android Debug Bridge) to clear the file artifacts. After uninstalling the app:
  1. Open a shell on your device - `adb shell`
  2. Change to the right direcotry: `cd /sdcard/Download/SpiderOak/.favorites`
    * Make sure you're in the right directory!  `pwd`
  3. Remove all the stored favorites: `rm -rf *`
  4. Now you can install the fixed version of the app, and un-favoriting should work from any section.

In working on this I noticed a refactoring of favorites and recents that would be a good start on reducing the ungainliness of FilesView.js. I've sketched it out in  #588.