Fix notes and discussions API path for epics

Currently to get notes or discussions for an epic, following API path is used: api/v4/groups/9970/epics/9/discussions

It should be: api/v4/groups/9970/-/epics/9/discussions because for accessing epic's API path is api/v4/groups/9970/-/epics/9.

UPDATE: Instead of adding /-/ to notes&discussions API endpoints, we will rather remove (make optional) /-/ from epics and search API endpoints. The reason is that we do not use this delimiter in other API (as there is no reason for it).