Fix mobile homepage styles

Fix mobile homepage styles

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2263

• Re-introduce styles from homepage-mobile.less
• Remove extra blank space on right-side using .width-block() mixin
• Fix logo and text not being next to each other
• Fix unnecessary spacing between top navigation and intro content

Before (without homepage-mobile.less)	Before (with homepage-mobile.less)	After

Testing strategy

1. Visit http://localhost:5000/?redirect=no
2. Make sure homepage looks fine on desktop
3. Make sure homepage looks good on mobile