Correctly load glyphicons in dropdown tabset

This will insure it works in `self_contained: false` and also simplifies the CSS. Closes #1697

Cheatsheet for Glyphicons: https://glyphicons.bootstrapcheatsheets.com/

DESCRIPTION

@@ -1,7 +1,7 @@
 Type: Package
 Package: rmarkdown
 Title: Dynamic Documents for R
-Version: 2.16.1
+Version: 2.16.2
 Authors@R: c(
     person("JJ", "Allaire", , "[email protected]", role = "aut"),
     person("Yihui", "Xie", , "[email protected]", role = c("aut", "cre"), comment = c(ORCID = "0000-0003-0645-5666")),

NEWS.md

@@ -3,6 +3,8 @@ rmarkdown 2.17
 
 - `rmarkdown::render_site()` will now also look for `index.rmd` in addition to `index.Rmd` for custom site generator in YAML with `site:` (thanks, @kamalsacranie, #2409).
 
+- Icons in tabset dropdown are now correctly shown, including when `self_contained: false` (thanks, @cpsievert, #1697).
+
 rmarkdown 2.16
 ================================================================================
 

inst/rmd/h/default.html

@@ -321,25 +321,18 @@
   border-radius: 4px;
 }
 
-.tabset-dropdown > .nav-tabs > li.active:before {
-  content: "";
+.tabset-dropdown > .nav-tabs > li.active:before, .tabset-dropdown > .nav-tabs.nav-tabs-open:before {
+  content: "\e259";
   font-family: 'Glyphicons Halflings';
   display: inline-block;
   padding: 10px;
   border-right: 1px solid #ddd;
 }
 
 .tabset-dropdown > .nav-tabs.nav-tabs-open > li.active:before {
-  content: "";
-  border: none;
-}
-
-.tabset-dropdown > .nav-tabs.nav-tabs-open:before {
-  content: "";
+  content: "\e258";
   font-family: 'Glyphicons Halflings';
-  display: inline-block;
-  padding: 10px;
-  border-right: 1px solid #ddd;
+  border: none;
 }
 
 .tabset-dropdown > .nav-tabs > li.active {