pihole -r: Fix WebUI not resetting or installing correctly

[agent47nh]

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What does this PR aim to accomplish?:

This PR fixes the issue #5473

How does this PR accomplish the above?:

Added a condition that installs the web UI if it was selected for installation but was not installed before. Else, it simply attempts to reset the existing web UI repo.

Link documentation PRs if any are needed to support this PR:

No changes are required in the documentation.

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