Reset information about modified or added items in subscription context

[olli-gold]

References

• Fixes Subscriptions of Items are sending mails to non-subscribers #380

Description

This PR resets information about modified or added items in subscription context. (see #380)

Instructions for Reviewers

See #380 for more information about the bug, which is resolved here.

List of changes in this PR:

• This PR just adds clearing of the item cache for subscriptions

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & integration tests). Exceptions may be made if previously agreed upon.
• My PR passes Checkstyle validation based on the Code Style Guide.
• My PR includes Javadoc for all new (or modified) public methods and classes. It also includes Javadoc for large or complex private methods.
• My PR passes all tests and includes new/updated Unit or Integration Tests based on the Code Testing Guide.
• If my PR includes new libraries/dependencies (in any pom.xml), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• If my PR modifies REST API endpoints, I've opened a separate REST Contract PR related to this change.
• If my PR includes new configurations, I've provided basic technical documentation in the PR itself.
• If my PR fixes an issue ticket, I've linked them together.