New community invitation web page for new unified app

[degenrocket]

The current community invitation web page can be improved.

• Current Mobile App should link to a new app (currently, links to a legacy app)
• View channel in Status didn't work on linux (tried a few browsers, DM on Status for more details)
• Show QR code doesn't work with auto-dark-mode enabled (e.g, in Chromium-based browsers).
• Feature request: make the page respect my system theme (e.g., Dark mode)

Now, if both View channel in Status and QR code don't work, then how can a user join a community? It's actually very confusing.

Thus, I'd suggest to extract a pubkey from URL and add one more option with the text like:

or click *discover communites* then *join communities* and paste this pubkey:
zQ3shvLb2TmuRreZ57VCLynfX5ukoqR275M7DMJDsC8BZvabs <copy-icon>

Finally, the page has too much text, so I'd simplify it and use the right section as well. Here is a quick example:

And minimalist version:

For a mobile version you can just hide QR code and shrink the download section.

Credits: zpunks public test

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Transferred this issue from status-app repo.

[jrainville]

cc @felicio