Fix image handling for thumbnails, use `path()`

This PR does a couple of things:

1. Fixes image handling, at least relating to thumbnails and tests. More concretely, images are uploaded and thumbnails are generated when running spotseeker_server.tests.images.thumb.ImageThumbTest, for example. (NOTE: if you run these tests, you'll still see "Bad Request" output--this is working as intended, and is due to 1) expected failure tests and 2) the log level for the django.request module).
2. Migrates most URLs from the Django 1.x style url() syntax to the Django 2.x style path() syntax. There are still three routes using url(), and I left them that way because my brain is having a hard time parsing regex right now.

.gitignore is also updated to include the space_images directory, which now gets properly created because image uploading works. Never mind, I added that to the wiscout project, not spotseeker_server.

Please review: @NUWAN.KUMARASIRI @TREFILEK @bjsousa