Let users post longer text submissions

- [x] Add TextPlanet model
- [x] Add text field to create_star macro
- [ ] Make text field optional in create_star macro (e.g. for group page)
- [x] Add text field to create_star form
- [x] Attach submitted text to new Stars
- [ ] Adapt star template to show text attachment if available
- [ ] Prevent creating TextPlanet when text field is empty
- [ ] Check whether TextPlanet sync works
- [ ] Check whether db migration works