Add filters for submission metadata

Problem/Motivation

The Webform submissions page currently provides only a generic search. Submission metadata (e.g. remote IP address, user, created date) can be displayed and sorted, but cannot be filtered.

This makes it difficult to investigate spam, abuse or other submission-related incidents.

Steps to reproduce

1. Open a Webform with many submissions.
2. Display the Remote IP address column.
3. Try to filter submissions by IP address or other submission metadata.
4. Filtering is not supported.

Proposed resolution

Add filter support for submission metadata on the submissions page (e.g. remote IP address, user, created date, completed date, notes).

Remaining tasks

Implement metadata filters on the submissions page.

User interface changes

Add filter controls for supported submission metadata.

API changes

None expected.

Data model changes

None expected.