Make Collection endpoint more flexible

[mbennett-uoe]

At the minute, there are a bunch of assumptions about the manifest:

• Canvases are called either page_<x> or just <x> (where <x> is the index)
• Pages in the ALTO file are called page_<x> (tesseract default - maybe other software too)
• Therefore we can link ALTO page to the Canvas by reading the manifest, making a list of Canvases and using index referencing
• Canvas URIs have a static relationship to the Manifest URL - see #16

If any of these are not true, Collection search won't work. Whatever solution to #16 we use, it should make this issue relatively easy to solve. Probably just need to change the SOLR response->Canvas lookup to be dict based instead of list based.