Allow more fine grained filtering of messages

[penguinpee]

This is a follow up to / split off from #1138.

Currently, the only filtering possible is by using globs for topic patterns and built in severity levels. I'd like to be able to use regular expressions for filtering, allowing for more fine grained filters.

This was triggered by the lack of information in Copr messages (fedora-copr/copr/issues/3237) in attempt to at least have some notification, but I think it might be useful in general. Of course, the usefulness is largely influenced by the level of information in the messages.

Above the input field for filters it states Topic pattern:. I assume this corresponds to the subject of the message in case of e-mail notification. If so, allowing to filter on the body, preferably as a separate filter, will also improve versatility of filters. If not, some clarification on what part of the message the filter is applied to, will be helpful.

[LecrisUT]

What would be a nice interface is to try and query the service for what topics it provides and provide a drop-down selection for them. Otherwise it is too magical to figure out what the topic was for a specific notification until you actually get one, but even then the topic is not clearly displayed in either the email or matrix endpoints.

This would be useful for packit notifications when it picks up the role of fedora-ci since then it would have vastly different topics from koji scratch builds to testing-farm tests on bodhi.