Issue #456: Use of Django connection name rather than alias as lookup

[sdether]

This commit changes connection behavior from storing the database connection name to using the database alias mapped by SQL Explorer instead.

The reason for this change is two-fold:

1. Views take the connection name as input, allowing anyone who knows Django connection names to query those databases, even if SQL does not expose the connection directly.
2. Query stores the connection name, which means that if the Django connection name changes or a different connection should be used (for example, one with reduced permissions) the stored Query will either stop working or at least continue using the old connection

This change modifies ExplorerConnections from being a dictionary that proxies the Django connection dictionary to a dictionary-like object that uses EXPLORER_CONNECTIONS to lookup and validate the requested connection alias.

In addition all code that used to the EXPLORER_CONNECTIONS value now uses the key instead.

For backwards compatibility, a migration will back-populate the alias into Query instances (and fail if the mapping no longer exists), EXPLORER_DEFAULT_CONNECTION is re-written on start-up to use the alias in case it still uses the Django Connection name and ExplorerConnections will still accept a Django Connection name as long as that name is exposed by some alias in EXPLORER_CONNECTIONS.