adds three column submission layout and basic list styles

opentech/static_src/src/app/src/SubmissionsByRoundApp.js

...atic_src/src/app/src/components/SubmissionsByRoundList.js

...c/src/app/src/components/SubmissionsByRoundListHeading.js

...src/app/src/components/SubmissionsByRoundListHeading.scss

..._src/src/app/src/components/SubmissionsByRoundListItem.js

...tatic_src/src/app/src/components/SubmissionsGrid/index.js

...tic_src/src/app/src/components/SubmissionsGrid/style.scss

opentech/static_src/src/javascript/main.js

opentech/static_src/src/sass/apply/abstracts/_mixins.scss

opentech/static_src/src/sass/apply/abstracts/_variables.scss