Search: add content/user safety messaging to our search results

### Impacted plugin

Jetpack

### What

When search detects potentially self-harmful user behavior let's enable admins to do something about it.

### How

On Tumblr (and possibly soon on wp.com Reader p58i-bL8-p2) we have messaging that will display self help information when a person searches for certain terms (suicide is the most obvious case, but there are many).

I think we could/should add this as an option to Jetpack Search. I think the implementation could be to implement the terms we match against on the API side and then display a message at the top of search results based on what the user was searching against.

I think this could overlap with the implementation for pinning search results or adjusting search results (see #15666). Still needs more refinement.

No files or tests are identified. Start by reviewing Jetpack Search and the related pinning or search-adjustment discussion in #15666; clarify the matched-term set, API behavior, admin option, and the result-message requirements before implementation.

Written by the indexing model from the issue text.