Add icons to repo & use script to generate font (#518)

This uses an npm package to convert SVGs into an icon font, allowing us to
not step on each others' toes when adding new icons in branches and
simplifying the icon addition process. It also allows us to track changes to
the icons, and embed them directly as SVGs if we need to.

---------

Co-authored-by: Johannes Klein <johannes.t.klein@gmail.com>

Gemfile.lock

@@ -209,6 +209,7 @@ GEM
     memoist (0.16.2)
     mini_magick (4.11.0)
     mini_mime (1.1.2)
+    mini_portile2 (2.8.1)
     minitest (5.15.0)
     molinillo (0.8.0)
     multi_json (1.15.0)
@@ -217,10 +218,14 @@ GEM
     nap (1.1.0)
     naturally (2.2.1)
     netrc (0.11.0)
+    nokogiri (1.14.2)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     optparse (0.1.1)
     os (1.1.4)
     plist (3.6.0)
     public_suffix (4.0.7)
+    racc (1.6.2)
     rake (13.0.6)
     representable (3.2.0)
       declarative (< 0.1.0)
@@ -279,6 +284,7 @@ PLATFORMS
 DEPENDENCIES
   cocoapods (~> 1.11, >= 1.11.2)
   fastlane
+  nokogiri
 
 RUBY VERSION
    ruby 2.7.5p203