Creator: When replacing an image, the upload option is available

[ryelle]

Describe the bug
Users do not have permissions to upload media but when replacing an image, the option to upload is there. Trying to upload anything will return an error, which can be confusing since the option is there.

To Reproduce
Steps to reproduce the behavior:

1. Add or edit a pattern
2. Insert an image block (or media & text, etc), pick an image from the Openverse library
3. Click on "Replace"
4. You see the option to upload, or edit the image URL

Expected behavior

"Media library" and "Reset" should be the only available options. Ideally the image URL is not editable as well, to prevent users from inserting potentially non-CC0 images (see #494).

Screenshots