FEATURE: Support additional metadata in theme about.json (#6944)

New `about.json` fields (all optional):
 - `authors`: An arbitrary string describing the theme authors
 - `theme_version`: An arbitrary string describing the theme version
 - `minimum_discourse_version`: Theme will be auto-disabled for lower versions. Must be a valid version descriptor.
 - `maximum_discourse_version`: Theme will be auto-disabled for lower versions. Must be a valid version descriptor.

A localized description for a theme can be provided in the language files under the `theme_metadata.description` key

The admin UI has been re-arranged to display this new information, and give more prominence to the remote theme options.

lib/theme_translation_parser.rb

@@ -1,8 +1,10 @@
 class ThemeTranslationParser
+  INTERNAL_KEYS = [:theme_metadata]
   class InvalidYaml < StandardError; end
 
-  def initialize(setting_field)
+  def initialize(setting_field, internal: internal)
     @setting_field = setting_field
+    @internal = internal
   end
 
   def self.check_contains_hashes(hash)
@@ -22,6 +24,9 @@ class ThemeTranslationParser
 
     parsed.deep_symbolize_keys!
 
+    parsed[@setting_field.name.to_sym].slice!(*INTERNAL_KEYS) if @internal
+    parsed[@setting_field.name.to_sym].except!(*INTERNAL_KEYS) if !@internal
+
     parsed
   end
 end

lib/version.rb

@@ -1,4 +1,6 @@
 module Discourse
+  VERSION_REGEXP = /\A\d+\.\d+\.\d+(\.beta\d+)?\z/ unless defined? ::Discourse::VERSION_REGEXP
+
   # work around reloader
   unless defined? ::Discourse::VERSION
     module VERSION #:nodoc: