fix duplicated download screen in nav stack

[catreedle]

Contributor checklist

• This pull request is on a separate branch and not the main branch
• I have tested my code with the xcodebuild and swiftlint --strict commands as directed in the testing section of the contributing guide

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Description

• Fix confirmDownload in SelectionViewTemplateController
• Delete duplicated Nav Controller for InstallationVC in AppScreen.storyboard

Related issue

• Closes Navigation stack duplication between Download Language and Translation Source Settings #580

[github-actions]

Thank you for the pull request! 💙

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[github-actions]

Maintainer Checklist

The following is a checklist for maintainers to make sure this process goes as well as possible. Feel free to address the points below yourself in further commits if you realize that actions are needed :)

• Tests for changes have been written and the CI unit test, linting and formatting workflows within the PR checks do not indicate new errors in the files changed

• The CHANGELOG has been updated with a description of the changes for the upcoming release and the corresponding issue (if necessary)

[andrewtavis]

praise: Just checked and all's working very well here, @catreedle 😊 Amazing work deducing the bug, making the issue and solving the problem! Thanks for your efforts :) :)