feat(firebase_ui_auth): Add "Go Back" button to EmailLinkSignInView

Description

This PR adds a "Go back" button to EmailLinkSignInView. Other views and screens in firebase_ui_auth have a similar button already, but EmailLinkSignInView does not, so users can get stuck if they got there by mistake, mistyped their email address, etc. This button code was copied from SMSCodeInputScreen, so it should be consistent with other auth flows in the package.

Related Issues

Fixes #81

Checklist

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[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[x] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
[x] All existing and new tests are passing.
[x] I updated/added relevant documentation (doc comments with ///).
[x] The analyzer (melos run analyze) does not report any problems on my PR.
[x] All unit tests pass (melos run test:unit:all doesn't fail).
[x] I read and followed the Flutter Style Guide.
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Breaking Change

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[ ] Yes, this is a breaking change.
[x] No, this is not a breaking change.

firebase / FirebaseUI-Flutter