discourse: Add a proper plugin builder + a few initial packages

Some discourse plugins have Ruby dependencies and require a
specialized builder. This introduces a generic builder that can be
used whether the plugin has Ruby dependencies or not. It also adds a
set of pre-packaged plugins available through `discourse.plugins` and
provides an easy way to add more.

(cherry picked from commit 7671b90919af8271bdf7e07bb86d63216a2145d6)

pkgs/servers/web-apps/discourse/plugins/discourse-github/Gemfile.lock

@@ -0,0 +1,37 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    addressable (2.7.0)
+      public_suffix (>= 2.0.2, < 5.0)
+    faraday (1.4.2)
+      faraday-em_http (~> 1.0)
+      faraday-em_synchrony (~> 1.0)
+      faraday-excon (~> 1.1)
+      faraday-net_http (~> 1.0)
+      faraday-net_http_persistent (~> 1.1)
+      multipart-post (>= 1.2, < 3)
+      ruby2_keywords (>= 0.0.4)
+    faraday-em_http (1.0.0)
+    faraday-em_synchrony (1.0.0)
+    faraday-excon (1.1.0)
+    faraday-net_http (1.0.1)
+    faraday-net_http_persistent (1.1.0)
+    multipart-post (2.1.1)
+    octokit (4.21.0)
+      faraday (>= 0.9)
+      sawyer (~> 0.8.0, >= 0.5.3)
+    public_suffix (4.0.6)
+    ruby2_keywords (0.0.4)
+    sawyer (0.8.2)
+      addressable (>= 2.3.5)
+      faraday (> 0.8, < 2.0)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  octokit (= 4.21.0)
+  sawyer (= 0.8.2)
+
+BUNDLED WITH
+   2.1.4