Cleanup: don't hard-code styles into layouts

[chalin]

We should avoid using Bootstrap utility classes (or even raw CSS) to hard-code styling of elements inside of layouts. Instead define an appropriate td- prefixed class, and add the styling to the class. Otherwise, it becomes difficult for clients to customize element styles.

• Show breadcrumbs on mobile too #784
• Blog layout: streamline and fix styling RSS button #1193
• Search styling refactoring, and fix for offline search #1279
• [BSv5] Navbar: add inner container + cleanup #1422
• Fix link text and hover color contrast for all bgs #1525
• [BSv5] Fix blog posts list flex layout #1566
• More changes are coming!