Feedback Submissions: Store author email as metadata

An upcoming version of WP core will support a personal data exporter/eraser and [this commit](https://github.com/Automattic/jetpack/commit/d0e87bd#diff-ba2abc1889d79bcf7293e6748302450bR913) added support for it. Personal data in Jetpack contact form submissions is identified by looking for Jetpack Feedback posts authored by a specific email address.

While working on that I discovered that in order to identify Feedback submissions associated with a specific email address I'd have to search the entire `post_content` column for a text fragment containing the author's email address on a line by itself; i.e., the author's email address is not stored anywhere else. This is more expensive that it needs to be.

Ideally, we would begin storing the author's email address in Feedback submissions as metadata. This way a search by email address would be more accurate/efficient.

Perhaps, at some point (maybe as a part of this work) we should consider storing all of the fields collected by contact forms as metadata. I think the current approach is not as extensible as it could be. The current approach being that everything is added to the `post_content` column.

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Reminder: Please re-review D13087-code when this task is completed.

Start with the Jetpack Feedback submission flow and review commit d0e87bd, which added personal-data exporter/eraser support. Confirm that the submission author's email is stored as metadata and that personal-data lookup can use it accurately and efficiently; re-review D13087-code when complete.

Written by the indexing model from the issue text.