Feat join meeting as user group

When a meeting closes, the closing report can be filed with a list of organizations that attended. After that, this information appears on the meeting page as metadata (column on the right) under the title of Attending Organizations. I propose to: - Allow users with a `UserGroup` role of `:admin` or `:creator` to attend a meeting representing a **verified** `UserGroup`. - Until the meeting closes, and to attract the user interest, show registered `UserGroup` profiles on the meeting page as _Attending Organizations_. The PR is already done. Sorry about that! https://github.com/decidim/decidim/pull/5060