Add support for deep links to user profiles.

[timonus]

Following up from this Mastodon conversation, this pull request adds support for opening user profiles via Amaroq's URL scheme. URLs that do this take the form

amaroq://user/[fully-qualified-mastodon-username]

For example

amaroq://user/timonus@mastodon.technology
amaroq://user/gargron@mastodon.social

Here's a video of this in action: https://db.tt/2pCCYe1pcp

[timonus]

Ah, looks like the /search endpoint will resolve URLs for statuses and accounts. May be better to use that, but I think this PR still stands :)

[timonus]

🙌

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