FEATURE: Native theme support

This feature introduces the concept of themes. Themes are an evolution
of site customizations.

Themes introduce two very big conceptual changes:

- A theme may include other "child themes", children can include grand
children and so on.

- A theme may specify a color scheme

The change does away with the idea of "enabled" color schemes.

It also adds a bunch of big niceties like

- You can source a theme from a git repo

- History for themes is much improved

- You can only have a single enabled theme. Themes can be selected by
    users, if you opt for it.

On a technical level this change comes with a whole bunch of goodies

- All CSS is now compiled using a custom pipeline that uses libsass
    see /lib/stylesheet

- There is a single pipeline for css compilation (in the past we used
    one for customizations and another one for the rest of the app

- The stylesheet pipeline is now divorced of sprockets, there is no
   reliance on sprockets for CSS bundling

- CSS is generated with source maps everywhere (including themes) this
    makes debugging much easier

- Our "live reloader" is smarter and avoid a flash of unstyled content
   we run a file watcher in "puma" in dev so you no longer need to run
   rake autospec to watch for CSS changes

app/models/remote_theme.rb

@@ -0,0 +1,85 @@
+require_dependency 'git_importer'
+
+class RemoteTheme < ActiveRecord::Base
+  has_one :theme
+
+  def self.import_theme(url, user=Discourse.system_user)
+    importer = GitImporter.new(url)
+    importer.import!
+
+    theme_info = JSON.parse(importer["about.json"])
+    theme = Theme.new(user_id: user&.id || -1, name: theme_info["name"])
+
+    remote_theme = new
+    theme.remote_theme = remote_theme
+
+    remote_theme.remote_url = importer.url
+    remote_theme.update_from_remote(importer)
+
+    theme.save!
+    theme
+  ensure
+    begin
+      importer.cleanup!
+    rescue => e
+      Rails.logger.warn("Failed cleanup remote git #{e}")
+    end
+  end
+
+  def update_remote_version
+    importer = GitImporter.new(remote_url)
+    importer.import!
+    self.updated_at = Time.zone.now
+    self.remote_version, self.commits_behind = importer.commits_since(remote_version)
+  end
+
+  def update_from_remote(importer=nil)
+    return unless remote_url
+    cleanup = false
+    unless importer
+      cleanup = true
+      importer = GitImporter.new(remote_url)
+      importer.import!
+    end
+
+    Theme.targets.keys.each do |target|
+      Theme::ALLOWED_FIELDS.each do |field|
+        value = importer["#{target}/#{field=="scss"?"#{target}.scss":"#{field}.html"}"]
+        theme.set_field(target.to_sym, field, value)
+      end
+    end
+
+    theme_info = JSON.parse(importer["about.json"])
+    self.license_url ||= theme_info["license_url"]
+    self.about_url ||= theme_info["about_url"]
+
+    self.remote_updated_at = Time.zone.now
+    self.remote_version = importer.version
+    self.local_version = importer.version
+    self.commits_behind = 0
+
+    self
+  ensure
+    begin
+      importer.cleanup! if cleanup
+    rescue => e
+      Rails.logger.warn("Failed cleanup remote git #{e}")
+    end
+  end
+end
+
+# == Schema Information
+#
+# Table name: remote_themes
+#
+#  id                :integer          not null, primary key
+#  remote_url        :string           not null
+#  remote_version    :string
+#  local_version     :string
+#  about_url         :string
+#  license_url       :string
+#  commits_behind    :integer
+#  remote_updated_at :datetime
+#  created_at        :datetime
+#  updated_at        :datetime
+#