Misleading error display when creating stream rules while a message is already loaded

When adding stream rules, the user needs to load an example message to see whether the rule would match.
Once one message is loaded, the user continues adding rules. We don't call testMatch endpoint and show new rules as non-matching, which can be misleading.
## Expected Behavior

When a message is loaded all added rules show the correct matching status.
## Possible Solution

Call testMatch when a new rule is added and a message is already loaded.

## Steps to Reproduce (for bugs)

1. create a stream rule
2. load a message
3. add new rules that should match

## Context
https://github.com/Graylog2/support/issues/64