Add general theming capabilities

This commit adds the requisite settings and startup features to
enable integration of themes into the edX platform. It does not
yet provide hooks in any of the templates, but it does cause the
main `lms/static/sass/application.scss` file to `@import` a theme's
base Sass. Template hooks will come down the road.

CHANGELOG
---------
Define a new `MITX_FEATURE`, `USE_CUSTOM_THEME`, that when enabled,
can be used in templates to determine whether or not custom theme
templates should be used instead of the defaults.

Also define a new setting, `THEME_NAME`, which will be used to
locate theme-specific files. Establish the convention that themes
will be stored outside of the `REPO_ROOT`, inside the `ENV_ROOT`,
in a directory named `themes/`. `themes/<THEME_NAME>` will store
the files for a particular theme.

Provide a function, `enable_theme`, that modifies the template and
static asset load paths appropriately to include the theme's files.

Move the main LMS Sass file to a Mako template that conditionally
`@import`s the theme's base Sass file when a theme is enabled.

Add logic to the assets Rakefile to properly preprocess any Sass/
Mako templates before compiling them.

rakefiles/assets.rake

@@ -1,3 +1,36 @@
+# Theming constants
+THEME_NAME = ENV_TOKENS['THEME_NAME']
+USE_CUSTOM_THEME = !(THEME_NAME.nil? || THEME_NAME.empty?)
+if USE_CUSTOM_THEME
+    THEME_ROOT = File.join(ENV_ROOT, "themes", THEME_NAME)
+    THEME_SASS = File.join(THEME_ROOT, "static", "sass")
+end
+
+# Run the specified file through the Mako templating engine, providing
+# the ENV_TOKENS to the templating context.
+def preprocess_with_mako(filename)
+    # simple command-line invocation of Mako engine
+    mako = "from mako.template import Template;" +
+           "print Template(filename=\"#{filename}\")" +
+           # Total hack. It works because a Python dict literal has
+           # the same format as a JSON object.
+           ".render(env=#{ENV_TOKENS.to_json});"
+
+    # strip off the .mako extension
+    output_filename = filename.chomp(File.extname(filename))
+
+    # just pipe from stdout into the new file
+    File.open(output_filename, 'w') do |file|
+      file.write(`python -c '#{mako}'`)
+    end
+end
+
+# Preprocess all static assets that have the .mako extension by
+# running them through the Mako templating engine. Right now we
+# just hardcode the asset filenames.
+def preprocess_assets
+    preprocess_with_mako("lms/static/sass/application.scss.mako")
+end
 
 def xmodule_cmd(watch=false, debug=false)
     xmodule_cmd = 'xmodule_assets common/static/xmodule'
@@ -32,10 +65,20 @@ def coffee_cmd(watch=false, debug=false)
 end
 
 def sass_cmd(watch=false, debug=false)
+    # Make sure to preprocess templatized Sass first
+    preprocess_assets()
+
+    sass_load_paths = ["./common/static/sass"]
+    sass_watch_paths = ["*/static"]
+    if USE_CUSTOM_THEME
+      sass_load_paths << THEME_SASS
+      sass_watch_paths << THEME_SASS
+    end
+
     "sass #{debug ? '--debug-info' : '--style compressed'} " +
-          "--load-path ./common/static/sass " +
+          "--load-path #{sass_load_paths.join(' ')} " +
           "--require ./common/static/sass/bourbon/lib/bourbon.rb " +
-          "#{watch ? '--watch' : '--update'} */static"
+          "#{watch ? '--watch' : '--update'} #{sass_watch_paths.join(' ')}"
 end
 
 desc "Compile all assets"