Add an endpoint to warm up the cache.

[bkyryliuk]

The api would receive the dashboard id and warm up the cache.

• Refactor the explore method into separate html and csv endpoint
• Add get_viz to the Slice and refactore the explore endpoint to use it
• Implement warmup_cache_dashboard/<dashboard_id> endpoint for the dashboard
• Implement warmup_cache_table/<table_id> endpoint for the table

The viz creation should look like this in the explore view:

if slice:
    viz = slice.get_viz(form_data)
else
    viz = viz.viz_types[viz_type](form_data)

Cache update for the dashboard:

for slice in dashboard:
    viz = slice.get_viz(force=True)
    viz.get_json()

similarly for a table
Cache update for the dashboard:

for slice in table:
    viz = slice.get_viz(force=True)
    viz.get_json()

[mistercrunch]

I'd like to remove all references to self.slice from the viz object.

I think it's just a matter of passing the cache_timeout as its own argument, or bundling it along with the rest of form_data