Add warning when trying to use Discord CDN images as project cover images #172
Description
Discord is making their CDN private next month, and as such, any files hosted on their CDN will become inaccessible for some time. We already have backed up most Discord CDN files on the website, but users can still add Discord CDN files through the project editor.
We should warn the user about using Discord CDN images for their project cover images because of this.
A full featured upload system should be added in the future so we don't need to depend on external file hosting services at all, but this will work for the time being.