Add support for building flatpak images

[halkeye]

I use arch, so I can't use use the .deb packages, and building locally doesn't really give you the schema stuff needed to do sso logins properly

So looked it up, electron has a flatpak builder.

Super basic setup, builds the flatpak package, uploads it as normal, nothing super fancy, but should cover any os that isn't debian based.

Checklist

• I have read and followed the contributing guidelines at https://docs.mau.fi/bridges/general/contributing.html

[tulir]

https://www.electronforge.io/config/makers/flatpak sounds like it won't work without installing extra packages in CI. Might be better to just submit a package to flathub

Also, the proper flatpak package ID would be something like fi.mau.mautrix-manager or fi.mau.mautrix.manager

[halkeye]

Also, the proper flatpak package ID would be something like fi.mau.mautrix-manager or fi.mau.mautrix.manager

I can certainly update the config, i just took a random guess. I like the first one myself. I'll have to check, it might just be fi.mau, and the packageName is mautrix-manager, i set this up pretty late last night

https://www.electronforge.io/config/makers/flatpak sounds like it won't work without installing extra packages in CI. Might be better to just submit a package to flathub

Do you have a preference here? I can update the ci files to install those dependancies (act doesnt' give me an accurate indication on whats installed)

but going the flatpak route, it looks like someone would have to go in and update the tag each release
eg https://github.com/flathub/com.discordapp.Discord/blob/master/com.discordapp.Discord.json#L116

I was kinda thinking this app was kinda a one and done type app, not something you kept around forever to constantly need to do logins

But i can setup the initial flathub repo if thats the path you rather.