* Revert "Fix header contents moving when opening modal (#2131)"
* Fix header contents moving when modal opened/closed.
Conditionally apply the navbar-fixed-top class only when needed, so that we can take advantage of it without always having the navbar in position:fixed, as was done in the previous solution. That resulted in a clash with custom headers.
* Show header on refresh of scrolled page
Due to some magic in Mithril 0.1's context:retain flag, some DOM elements were cached across page reloads. Since that has been eliminated, if we refresh the page and we are scrolled down, the "affix" class which makes the header fixed (and as a result, visible) isn't applied until the first scroll. We fix this by running ScrollListener.update() immediately to set initial navbar state.
- Change preview icon to Font Awesome Regular instead of Solid
- Give the preview button a tooltip
- Change submit button icon to paper plane (shown on mobile)
- Tweak mobile header button touch target
- Scroll overflowing text editor controls horizontally
* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory
