Provide UI for configuring devappserver storage path location(s)

In #1245, we add support for identifying if two devappservers have conflicting resource requirements, such as the server port, admin port, and the base storage location.

The base storage path normally contains the files from the various App Engine services (datastore, blobstore, search index, and logs). This base storage paths is set with `--storage_path` and defaults to `$TMPDIR/appengine.${APPID}.${user.name}`. The _APPID_ is specified with the `-A` flag, and defaults to _None_. The individual stores can also have explicit paths set too, which can be useful for debugging purposes such as to test with canned data.

We could make the storage paths not conflict by allowing the user to either:
- specify an _APPID_
- specify the base storage path (passed to `--storage_path`)
- specify the various explicit storage paths (`—blobstore_path`, `--datastore_path`, `--logs_path`, `--prospective_search_path`, `--search_indexes_path`)
It might even make sense to set the default to be kept in the per-Server directory so that the results persist between server restarts.