Going Bagless: changing out problemBag

The problemBag is confusing (with respect to multiple problems being loaded), because it seems as though it is reloaded each time, making each draw random (with replacement) rather than shuffled (without replacement). If it’s not really important to be precise with the weighting (that of of 10 draws, something with probability .30 is chosen exactly three times), then the following suffices:
```
    problem = problems.eq(Math.floor(Math.random() * typeIndex.length));
```
To make the exercises compatible:
- Changed all exercises with `data-weight` equal to some decimal to the corresponding integer.
- Deleted some exercises with `data-weight=“0”` (https://github.com/Khan/khan-exercises/commit/ece572aa0a283b5de3149750aeaf0306bf1b16e0#commitcomment-4769554)
- Some exercises with `data-weight=“0”` used it as a template to reduce redundancy. Since this is clever, I added the one line necessary to allow it.