Publish nextcloud talk flatpak package on Flathub

[pktiuk]

Is your feature request related to a problem? Please describe.

Distribute nextcloud talk flatpak package via flathub, because nextcloud-talk desktop lacks easy way of installing and updating.

Additional context

Linked with issues: #848 , #257

In comment #257 (comment) @ShGKme stated:

Are there plans to publish the flatpak at flathub?

Yes, if we go with Flatpak

[SyndicatedPillbug]

This would be really nice.