Refactor session model/save

[mgrauer]

Right now we save the session in a few ways

• add/remove a dataset enable the save button
• save basemap properties through target icon
• save basemap name/description through session edit
• save basemap center through session edit

We should unify save methods.

Other changes:

• currently save session in json file, instead we should use item metadata
• change session model to extend minervamodel, use the item metadata API in minervamodel

Decide if saving the session saves session data or app state (config/layout) or just as one big blob.

[danlamanna]

Additionally, use Backbone get/set. I think (but am not positive) that this event is never executed.

[mgrauer]

Be sure to bring back the ability to save the baselayer properties somehow, since they were removed in #226.

[mgrauer]

Address this comment and that comment.

[mgrauer]

Functionality related to an individual session is

• name
• description
• basemap
• choice of basemap
• zoom level
• center
• currently visualized datasets
• any color mapping/filtering/adapter properties of the visualized dataset
• order of datasets in the stack
• opacity of datasets

[mgrauer]

Sessions have been fixed to save in item metadata instead of a json file, in #319.