feat: add database dropdown to dashboard import (#10118)

* 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

superset/models/dashboard.py

@@ -247,7 +247,7 @@ class Dashboard(  # pylint: disable=too-many-instance-attributes
 
     @classmethod
     def import_obj(  # pylint: disable=too-many-locals,too-many-branches,too-many-statements
-        cls, dashboard_to_import: "Dashboard", import_time: Optional[int] = None
+        cls, dashboard_to_import: "Dashboard", import_time: Optional[int] = None,
     ) -> int:
         """Imports the dashboard from the object to the database.
 
@@ -311,6 +311,10 @@ class Dashboard(  # pylint: disable=too-many-instance-attributes
         # copy slices object as Slice.import_slice will mutate the slice
         # and will remove the existing dashboard - slice association
         slices = copy(dashboard_to_import.slices)
+
+        # Clearing the slug to avoid conflicts
+        dashboard_to_import.slug = None
+
         old_json_metadata = json.loads(dashboard_to_import.json_metadata or "{}")
         old_to_new_slc_id_dict: Dict[int, int] = {}
         new_timed_refresh_immune_slices = []
@@ -332,8 +336,8 @@ class Dashboard(  # pylint: disable=too-many-instance-attributes
             new_slc_id = Slice.import_obj(slc, remote_slc, import_time=import_time)
             old_to_new_slc_id_dict[slc.id] = new_slc_id
             # update json metadata that deals with slice ids
-            new_slc_id_str = "{}".format(new_slc_id)
-            old_slc_id_str = "{}".format(slc.id)
+            new_slc_id_str = str(new_slc_id)
+            old_slc_id_str = str(slc.id)
             if (
                 "timed_refresh_immune_slices" in i_params_dict
                 and old_slc_id_str in i_params_dict["timed_refresh_immune_slices"]