Show PurchaseUrl if present in manifest

[Trenly]

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This does not take into account any of the entitlement checking for the MSStore source; The only change this should have is to Pre-Indexed and Rest sources where, if the PurchaseUrl is present in the manifest, the URL will be displayed.

As part of this I have also taken the many if statements for checking if fields are empty and moved them into a function for ease of readability.

Loosely related to #1345

Microsoft Reviewers: Open in CodeFlow

[JohnMcPMS]

/azp run

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[ryfu-msft]

Looks good to me, will defer to @yao-msft for sign-off and merge

[yao-msft]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).