Unify pseudo-element notation with double colons

https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements
Add decent gallery styles
Captions won't be shown on sixth column, either
Improved search form with Flexbox, changed entry-meta to entry-footer
Updated Modernizer and added support for widget changes in customizer
Search form max-width is global now
Improved display of social icons with Flexbox
Improved footer text layout
Expanded max width of search bar
Form fields for new users display inline on screens larger enough
Displayed images as block elements in galleries
Simplified gallery caption styles
Again, simplified gallery styles
Improved layout for comment form, hid allowed form tags by default and added setting to display them, image attachment pages
Provided more room for allowed-form-tags
Moved comment reply button to right side, reversed in RTL mode
Added RTL styles for author card
Updated selectors for custom colors
Updated to Grunt 0.4.3
Submenu shadows are more clear