Persist active portal in browser cache and in global app state

[jps327]

Persist active portal in browser cache and in global app state keep track of global switch.

Fixes #237

Test plan

• Change the active portal
• Select a dataset
• Click on the "Explore" nav icon
• The home page should return to the previously selected portal. It should not default back to NYC.
• Now switch to all portals
• Click on a dataset
• Click on the "Explore" nav icon
• The home page should remember that we were looking at global portals
• Remove the global switch. We should now be back to the previously selected portal. Not defaulting to NYC.
• Add the global switch back on.
• Refresh. We should still be on global view.