Check links

adds in explicit link structure for podcast pages. also adds in inference for where images are, based on agreed format. 

link structure should follow 


/podcast

/podcast/episode1
/podcast/episode1/segment1
/podcast/episode1/segment2
/podcast/episode1/segment3

cdn.elifesciences.org/podcast/episode1.jpg
cdn.elifesciences.org/podcast/episode1-segment1.jpg
cdn.elifesciences.org/podcast/episode1-segment1.jpg

/podcast/episode2
/podcast/episode2/segment1
/podcast/episode2/segment2
/podcast/episode2/segment3

/podcast/episode3
/podcast/episode3/segment1
/podcast/episode3/segment2
/podcast/episode3/segment3