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.