Quote showname when it is being used in url parameter (#3419)

* Quote showname when it is being used in url parameter

* Revert "Quote showname when it is being used in url parameter"

* Use Mako expression filtering to ensure that showname is quoted properly

Also use 'loading_show_name' as alt value in img tags instead of
'loading_show_id', as it is more concise in the case where
'loading_show_id' is set the show's path.