Order slices/tables for import/export

[smacker]

Example of changed order: https://github.com/src-d/sourced-ui/pull/212/files#diff-8f99a0ab0cdfba56f87162db3d1757fa

We need to investigate why do they change the order.
Probably we need to do something similar to apache/superset#7728 or sort by some other value that doesn't change.