Ensure website is aware of the new custom tag

[barthalion]

Hmm, yeah, I guess I must have implemented it first with <custom>, then realized it's not what appstream-glib does. @razzeee we're going to have to switch backend and frontend to use the new field too at some point.

Originally posted by @barthalion in flathub-infra/flat-manager-hooks#101 (comment)

[razzeee]

Worst case this needs to be done before libappstream, but I'll try to get to it this week

[barthalion]

Yeah, we should do that earlier. I can handle the backend part.

[razzeee]

I guess we also handle both https://github.com/flathub-infra/website/blob/main/backend/app/utils.py#L216

guess custom will just overwrite stuff from metadata, which should be fine

[barthalion]

Yeah, I guess we're backwards compatible after all.