[4.0] [plg_privacy_user] Convert to prepared statement

[HLeithner]

Summary of Changes

Updated SQL queries to prepared statements and made some cleanups around the queries.

Testing Instructions

Use the plugin in all ways you can think of.

Create users
Delete user
Export privacy reports

Expected result

Nothing changed.

[brianteeman]

2 unrelated files

[joeykrop]

I have tested this item ✅ successfully on ea60065

Tested as much as possible.

Applying for reports, confirming and receiving also worked.

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[crommie]

I have tested this item ✅ successfully on ea60065

Played around with users. Works as expected.

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[waader]

I have tested this item ✅ successfully on a4e188c

Tested with postgresql.

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[deepa-g]

I have tested this item ✅ successfully on 2ed0cbd

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[vijaykhollam]

I have tested this item ✅ successfully on 2ed0cbd

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[pravinTek]

I have tested this item ✅ successfully on 2ed0cbd

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[alikon]

RTC

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[wilsonge]

Thanks!