[Settings] Apply default HTML to the editor via templates

- Added a template HTML5 file `html5.html` to be used as default HTML in the editor
- Updated package.json to use gulp-html2js
- Added a new task to gulpfile "html-templates"  for converting html templates to JS files and including it as a prerequisite in the "source" task
- updated settings HTML to import the html5 template
- Controller updated to use $templateCache to retrieve the html5 template and then apply it as a default value for the editor