Merge branch 'feature_multiple_default_sort' of https://github.com/stepsame/flask-admin into sqla-sort-multiple

Fix #1715 python3 exception has no attribute 'message'

Add missing class_name to bootstrap 2/3 menu items

This is part of the code, but wasn't in the template.

https://github.com/flask-admin/flask-admin/blob/v1.5.2/flask_admin/menu.py#L133

Error in Ukrainian translate 

Change Ukrainian translate error

Upgrade leaflet

Add file-admin based on Azure Blob Storage

Currently the unit tests for file-admin are tied to the local
implementation. However, all other implementations of the file-admin
(e..g S3) should maintain the same invariants as the local one. As such,
this change makes it easier to copy the local file-admin tests for other
storage backends.