feat: add fork remote command

[karolzwolak]

PR Description

The command allows you to quickly add a new 'fork' remote with
replaced owner in the origin URL of the selected remote.

For example:
given url: https://github.com/jesseduffield/lazygit.git
and username: karolzwolak
adds a new remote with url: https://github.com/karolzwolak/lazygit.git

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

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