Include pipeline rule content/source matches when searching on `Pipeline Rules`

## What?

Searching on the `Pipeline Rules` page (i.e. `/system/pipelines/rules`) should include matches on pipeline rules' content/source.

## Why?

This would make it easier to:
* identify use of certain functions (for reference when developing new rules, finding deprecated functions [e.g. `remove_field` in #19287], etc.)
* find other useful content (e.g. `TODO`s, system identifiers, IPs / CIDR ranges, etc.)

The current approach I use is to dump all pipeline rules via the API and search that content, but that's a bit tedious (and without parsing the JSON the content has `\"`, `\n`, etc. all over the place).

## Your Environment

* Graylog Version: `6.1.5`