Configrationable repository url (#10600)

* config: Add GITHUB_REPOSITORY for repository name

* config: Add SOURCE_BASE_URL for repository url

* Show source_url and repository name on getting started

lib/mastodon/version.rb

@@ -33,11 +33,11 @@ module Mastodon
     end
 
     def repository
-      'tootsuite/mastodon'
+      ENV.fetch('GITHUB_REPOSITORY') { 'tootsuite/mastodon' }
     end
 
     def source_base_url
-      "https://github.com/#{repository}"
+      ENV.fetch('SOURCE_BASE_URL') { "https://github.com/#{repository}" }
     end
 
     # specify git tag or commit hash here