* feat: add database dropdown to dashboard import
Currently, when importing a database from a JSON file, the process
looks at the database name from the source (the info is in the file)
and matches the datasources to that name. If no database by that name
exists, it simply fails.
With this PR, we add a database dropdown that allows the user to specify
which databases the datasources should target as the get upserted.
I want to stress that the code in this area is not in a great shape,
and that the challenge of serializing/deser the nested objects is
challenging, but that there should be a much better way to do this.
One of the improvement (out of scope for this PR) that would allow to
simplify those import/export would be to use UUIDs for
importable/exportable objects.
Another identified issue is the indirections between
`utils/import_expor_{model}.py` on top of `{Model}.import_object`. Not
addressing that here.
Next topic is the MVC stuff. Decided to stick with it for now as this is
more of a [obious missing feat:] than a rewrite.
* isort \!? 0%^$%Y$&?%$^?%0^?
* fix tests
* pre-committing to py3.6
* address dpgaspar's comments
* revert isort
