Forum thread & proposal labeling

[chrlschwb]

Background: https://github.com/Joystream/pioneer/issues/4497

• [ ] Label Metaprotocol in Forum Thread and Proposals #4830
• [ ] Create and assign labels #4831
• [ ] Functionality for adding/editing/hiding/unhiding labels
• [ ] Showing labels on forum threads and proposals
• [ ] Functionality to add labels to forum threads and proposals
• [ ] Search function for labels