Add getQueryApplicationClass() to *TransactionQuery.php classes

Summary:
Add getQueryApplicationClass() to all *TransactionQuery.php classes similar to other *Query.php classes having the same function, and make the parent function in PhabricatorApplicationTransactionQuery.php abstract.

In the future, this will enable excluding transaction query results based on their underlying application (for example if an application has been uninstalled) to mitigate the problem of overheated search results. See https://we.phorge.it/T15642 for context.

The only callers of getQueryApplicationClass() are in src/infrastructure/query/policy/PhabricatorPolicyAwareQuery.php and src/applications/policy/__tests__/PhabricatorPolicyTestCase.php.

See T15642

Test Plan:
Patch changes only one existing code place, thus check if related pages still work as expected:

• Go to http://phorge.localhost/feed/
• Go to http://phorge.localhost/feed/transactions/
• On http://phorge.localhost/feed/transactions/ , click Edit Query and set Object Types to Application etc.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15642

Differential Revision: https://we.phorge.it/D25500