PR - [SUP-238] display the max slug length

- [ ] PR
- [ ] QA

A pull request has been created for this change in all applications. Please treat them as a single change:

- [supporter](https://github.com/everydayhero/supporter/pull/125)
- [command_center](https://github.com/everydayhero/command_centre/pull/1074)
- [core](https://github.com/everydayhero/core/pull/460)
- [admin](https://github.com/everydayhero/admin/pull/120)