… as lookup

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`.