Add Shot model. Hack migrations to work around MySQL bug

I know,  deleting migrations is horrible  - but no-one is  going to be
using this branch in production...
Add MDSplus shot manager, remove reference to shot node
Add base Shot manager class

Data modules are expected to subclass this base Shot manager class.
The base class provides methods for fetching the first and last shot
numbers, and for finding the next and previous shots (not necessarily
shot +/- 1, in case there are missing shots).
update templates
new management command to add new shots
custom H1 shot manager which is aware of shot timestamps
remove reference to H1DS_DATA_MODULE in templatetags
add shot serializer
explicity add shot in url regex
util to locate backend shot manager
use rest framework views, remove old views