Fix CSV user import with UTF-8 BOM and improve error handling

[rgaviras]

This fixes CSV user imports for UTF-8 files with BOM when the first header is not quoted.

The issue occurs with valid CSV files like:

name;firstname;lastname;email;password

When the file starts with a UTF-8 BOM, the importer reads the first column as \ufeffname instead of name. It then tries to resolve a dynamic setter for that field, fails to find the
expected CmsUser#setName(String), and the import can fail with unclear behavior.

This PR:

• removes an optional UTF-8 BOM from CSV headers and values, with both quoted and unquoted headers;
• keeps the existing dynamic CmsUser#setXxx(String) reflection behavior;
• keeps support for deployment-specific/custom CSV columns by storing fields without a matching setter as additional user info;
• validates the mandatory name column and empty user names with clear errors;
• handles incomplete rows without IndexOutOfBoundsException;
• preserves and logs the real cause of setter invocation failures;
• removes silent catches in the affected import flow;
• sends notification emails only after the user has been created;
• reports secondary failures for group, role, settings, or email operations without aborting the whole import;
• avoids logging password values;
• adds focused tests for BOM handling, variable columns, incomplete rows, missing name, and password fallback behavior.

Validated with:

• ./gradlew compileJava
• ./gradlew testSingle --tests org.opencms.ui.apps.user.TestCmsCsvImportUtils

Also manually tested on OpenCms 21.0.1 by replacing opencms.jar in a container.