Allow groups to be invited to topics

[xrav3nz]

Interestingly enough, this is not due to rate limiting at all: the invites are rate limited message is hardcoded and will be shown regardless.

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The real cause - groups can only be invited to private messages:

app/controllers/topics_controller.rb

Do we want to either:

1. Disable being able to select a group altogether
2. Allow groups to be invited to topics