#5666 Add initial support for Thunderbird (improved setup flow)

[martgil]

This PR adds initial support for the Thunderbird desktop email client and the specific goal is to improve the set up flow accordingly.

close #5666
issue #5667

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Tests (delete all except exactly one):

• Difficult to test - (changes are not readily completed for tests)

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[tomholub]

How is this looking so far? This looks like it could close #5665 nearly as is. Then you could work on the followup issues in separate PRs. It's always better to make PRs smaller if possible to make progress easier.

One thing I noticed is that the content_scripts/webmail folder already on master is quite disorganised and not so easy to work on to add Thunderbird (or any other webmail) support. I country try and refactor it a little bit myself, and that could make it easier for you.

[tomholub]

When #5748 is merged, you can follow #5748 (comment) to close #5665 with this PR, then start working on #5666 in next issue. Let's address these one by one instead of trying to do all at once.

[martgil]

I understood completely, Tom. I'll be following your advice instead of making them work all at once.

[martgil]

Hi @sosnovsky - This one is ready for review. Thank you!

[sosnovsky]

Hi @martgil, can you please describe (or maybe make screenshots) of functionality which can be tested in this PR?
I installed FlowCrypt extension in Thunderbird, configured it and now can see FlowCrypt button on toolbar.
But when opening Encrypted inbox, it loads just empty page.