Bulk upload of media assets

**Is your feature request related to a problem? Please describe.**
I want to upload a number of photos that belong to a album. I want to upload them all at once.
Ideally, I want set some common properties like a common tag as described in #370.

**Describe the solution you'd like**
I want to be able to select multiple files in the uploader.
I probably also want to be able to drag and drop multiple files into the uploader field.
Ideally, I can set some properties like a tag in the uploader.

The issue names no files, tests, or entry points. Start by locating the existing uploader and album/tag handling, then determine how multiple selections, drag-and-drop, and shared properties should work; done means multiple media assets can be uploaded together with common properties applied.

Written by the indexing model from the issue text.