BulkImports: subrelation import is too slow [PoC]

Problem

Subrelations we currently import are epic events and epic award emojis. I did a test of importing 6k epics (similar number of epics to gitlab group) and the overall import was extremely slow (1h+).

This is due to the fact that we do 1 network request per epic and iterating through them one by one. This is a very time consuming process and as the amount of subrelations we import grows it will only get slower.

Because of this, we need to find a better way to import subrelations.

Up until this point we were trying to avoid dealing with nested paginated subrelations but perhaps it's time we address it for epics and for any other future relation that has subrelations as well.