handle theme names containing a `.` correctly.

zpublisher puts them into another namespace so manage_saveBundlesFortheme won't find theme.

eg ``REQUEST.form`` is::

  {'mappings.my': {'theme': ['jquery', 'mytheme']}, 'mappings': {'Plone Classic Theme': ['jquery', 'default'], 'Plone Default': ['jquery', 'default'], 'Sunburst Theme': ['jquery', 'default']}}

instead of::

  {'mappings': {'my.theme': ['jquery', 'mytheme'], 'Plone Classic Theme': ['jquery', 'default'], 'Plone Default': ['jquery', 'default'], 'Sunburst Theme': ['jquery', 'default']}}

we work around this by replacing dots with `___` in the form handling.