Prepend theme templates to lookup paths

Currently theme templates are appended to the lookup paths, making
it impossible for theme templates to totally override default
templates. Prepend the paths to theme templates instead.

lms/envs/common.py

@@ -751,9 +751,10 @@ def enable_theme(theme_name):
     # Calculate the location of the theme's files
     theme_root = ENV_ROOT / "themes" / theme_name
 
-    # Include the theme's templates in the template search paths
-    TEMPLATE_DIRS.append(theme_root / 'templates')
-    MAKO_TEMPLATES['main'].append(theme_root / 'templates')
+    # Prepend the theme's templates to the template search paths so
+    # that themes can override templates on an as-needed basis
+    TEMPLATE_DIRS.insert(0, theme_root / 'templates')
+    MAKO_TEMPLATES['main'].insert(0, theme_root / 'templates')
 
     # Namespace the theme's static files to 'themes/<theme_name>' to
     # avoid collisions with default edX static files