Fixes JIRA LMS-203

Feature/cdodge/soft delete assets

allow for an optional throw_on_not_found on contentstore().find() so we can clean up caller logic in the tests

add new collection configuration to support a 'trashcan' for assets. Update class factory to allow callers to specify if they want the default set of assets or the trashcan

Added info to changelog for recent pull requests.

Addtional comments for YouTube API bug.

fixing segment io settings bug

* Moving to the auth tokens section
* Removing the feature flag logic since all features are loaded earlier
```
    for feature, value in ENV_TOKENS.get('MITX_FEATURES', {}).items():
        MITX_FEATURES[feature] = value
```

Update to look for segment io key in auth data.

Simplify calc.py; add trig/other functions

Default to unicode for sass compilation.

Replace /faq route in LMS urls

remove compiled coffeescript files before recompiling with rake

Add additional events for tracking

force-refresh youtube iframe when changing speeds back to 1.0 in firefox

Valera/no autoplay videoalpha studio

Moved the definition of autoplay attribute to videoalpha_module.py file. Changed double negative if statement for autoplay check to simple if.

Copied autoplay attribute from video xmoudle to videoalpha. Now students will have the video autoplaying, and in Studio it will be pause by default.

If Video Alpha is playing a YouTube video in Firefox in HTML5 mode, then when switching to speed 1.0 it will simply reload the iframe, instead of using the built-in method to change the speed. The fixes the bug temporarily, while the YouTube team fixes the bug internally in the API. Commented additions to fix YouTube firefox speed 1.0 switch bug.