Batch upload images with XML or CSV uploads

[1mi]

For now there is an option to batch upload observations from XML and CSV files. However the images still have to be uploaded one by one for each observation.
It would be nice if we could batch upload images together with the observation files in one go.

The implementation could be as simple as adding another "Browse images" button under Browse file button or just add a Drag-and-drop area like it is for now for an individual image upload.

Whenever the user uploads some images DeepskyLog should warn if some images were not referenced in the XML or CSV file. Optionally it can display a list of image filenames that were not uploaded.

Finally when the user clicks "Import" matched images should be uploaded to the associated observation, and any extra not-referenced images should be deleted from the database.

XML files based on OAL already have a place for referenced images. CSV files would need an additional column for this.

[github-actions]

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[WimDeMeester]