Custom layout feature

- doing away with "container" and only using "scrollContainer" now
- adding `images.js` to list of dependencies for unit testing widget
- added `images-spec.js` for unit testing the Images module responsible for loading spreadsheet images
- new content.js which is a revision of table.js to handle all accounts of custom layouts
- adding images.js to handle the loading of all image and qr code images
- widget HTML updated to include import of images.js and inline css
- removed all sass files and "widget-styles" gulp task to now put widget table layout css inline in the HTML
- refactored "spreadsheet.js" to improve its flow and logic and create/initialize the content

Settings
- Added tests to cover layout UI
- settings HTML updated to change `ng-show` to `ng-if` for `url-entry` directive to avoid the problem of it validating when not being displayed
- adding a `config` module in config files to define value `defaultLayout` for settings app, the value is temporary for testing purposes
- controller logic updated to add `layoutURL` to `params` and `customURL` to `additionalParams.layout`
- settings app updated to use new `config` module