Make notification checkbox more obvious

- Clarify checkbox description wording
- Wrap checkbox and envelope icon in div to make it stand out
- Added styling for wrapper div
- Wrap label element around div to allow clicking the envelope to activate the checkbox
- Indented HTML and fixed if wrapping issue

@marcotuts & @gwprice - here's the new PR to make the checkbox more obvious without moving it. (This PR is a result of the discussion on tho other PR: #2383)

![screen shot 2014-01-31 at 11 02 31 am](https://f.cloud.github.com/assets/3364609/2053596/4eecf6e0-8aaa-11e3-9182-f3d4f4dc3acb.png)

![screen shot 2014-01-31 at 11 02 40 am](https://f.cloud.github.com/assets/3364609/2053601/5ff56e04-8aaa-11e3-80b1-1898cfa14c43.png)