Fix icons

- relying upon glyphicons as part of bootstrap, but the Sass for that doesn't compile properly in dart-sass because it uses `url(font-path(path))`
- instead copy the font-face declaration from bootstrap into our code, overridding the original with this one, which fixes the icon paths and the icons
- see similar problem fixed in content-tagger: alphagov/content-tagger#1768

app/assets/stylesheets/legacy_layout.scss

@@ -23,3 +23,9 @@
 @import "legacy/style";
 @import "legacy/versions";
 @import "legacy/glossary";
+
+@font-face {
+  font-family: "Glyphicons Halflings";
+  src: url("bootstrap/glyphicons-halflings-regular.eot");
+  src: url("bootstrap/glyphicons-halflings-regular.eot?#iefix") format("embedded-opentype"),url("bootstrap/glyphicons-halflings-regular.woff2") format("woff2"),url("bootstrap/glyphicons-halflings-regular.woff") format("woff"),url("bootstrap/glyphicons-halflings-regular.ttf") format("truetype"),url("bootstrap/glyphicons-halflings-regular.svg#glyphicons_halflingsregular") format("svg")
+}