Refactor compare view

[koesie10]

This makes a start on refactoring the compare view to be more maintainable and extensible. The main changes are:

• Extracting a findResultSetNames function from findCommonResultSetNames
• Renaming findCommonResultSetNames to findResultSetsToCompare
• Caching BQRS schemas instead of reading them when changing result sets

It's easiest to review this commit-by-commit.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

LGTM, thanks for the individual commits!