Add Thunderbird Beta

[snoweuph]

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Is your feature request related to a problem? Please describe. no

Describe the solution you'd like Thunderbird Android Beta (I want the Thunderbird branding and use the Beta)

Additional context Currently Theres just the K9 Mail app, which in the Futurte will become Thunderbird. I want to use the Beta, to have the new Branding and write Bug reports