Use NOTION_HOME for test sandbox

[dherman]

The sandboxing logic we use for testing is a bit messier than it could be. I think we can clean it up by eliminating the NOTION_SANDBOX env var and not changing HOME or USERPROFILE, and just use NOTION_HOME as the main hook for sandboxing.

Rough plan:

• don't change HOME or USERPROFILE -- don't need to isolate behavior of tools we're delegating to
• use NOTION_HOME as only sandboxing hook
  • unix: replaces ~/.notion
  • windows: replaces C:\Users\$username\AppData\Local\Notion
• test sandbox should just set NOTION_HOME to $unique/.notion on all platforms
• path::mod should define notion_home
  • if NOTION_HOME is set, use that
  • otherwise use default_notion_home()
• path::$os should define default_notion_home
  • unix: ${dirs::home_dir()}/.notion (see dirs::home_dir())
  • windows: ${dirs::data_local_dir()}/Notion (see dirs::data_local_dir())
• all the path functions should root off of notion_home()
• eliminate NOTION_SANDBOX

It might be best to wait until #181 is done to implement this, since that touches these files a fair amount.