- Renamed `clientapi/auth/types` to `clientapi/auth/authtypes` for the same
  horrible namespace clashing reasons as `storage`.
- Factored out `makeAPI` to `common`.
- Added in `makeAuthAPI`.

Previously, all database stuff was under the helpfully named
package 'storage'. However, this convention is used throughout all
of dendrite, which will clash the moment we want to add auth to all
the CS API endpoints. To prevent the package name clash, add
sub-directories which represent what is being stored so the final
usage ends up being:

```
func doThing(db *storage.SyncServerDatabase, authDB *accounts.Database)
{
    // ...
}
```