Add Meta and OffTopic categories on Forum and "solved" plugin for Discourse

It has been time since a user asked for a Meta category in the Forum, to speak about the forum itself: https://community.e.foundation/t/feature-proposal-add-meta-category/9264

Also, it would be good to have the OffTopic category. I missed it many times, as in the forum is really interesting and wise people and I wanted to get their help.

Finally, adding the "solved" plugin for Discourse (the Forum), which lets post-creator remark the answer that solved the question, would ease a lot the workflow of the Forum.