Add settings for mirroring copied images to the user's server

[wheelercj]

Related: #79 and #80

When copying an image to be pasted into a remote editor like GitHub, Stardown only gets the image's URL, not the image itself. This works, but will eventually stop working when the image is no longer served by that URL.

Those using remote editors like GitHub may want Stardown to automatically mirror images and use the mirrored image URLs in image markdown so that image markdown continues working longer.

Some users may want to use an image mirroring service that's external to Stardown but can integrate with Stardown. This way, we would not have to moderate the images.

[!note] Use of this feature will require a paid subscription.

[wheelercj]

ImgBB is a popular image hosting service with an API and a free tier. This might be a good starting point.

[wheelercj]

• immich-app/immich: High performance self-hosted photo and video management solution.
• Talal-A/obsidian-immich -- "This plugin allows users to easily insert images from their self-hosted Immich instance into their Obsidian notes."