Warn about github usernames and app names with hyphens or underscores

[hanaral]

What Happened

Flatpak, due to it following freedesktop and dconf spec, cannot have hyphens in the username part of the reverse-DNS, so flatpak recommends using underscores. However, GitHub does allow hyphens in names, but doesn't allow underscores - so you can see the issue. Here is a perfect example of this problem already with an AppCenter developer

Expected Behavior

Dashboard warns that hyphened names should use underscores instead in apps