Feature Request: Base class features to be made possible to add to custom classes

[knappetroll]

Motivation / Use Case

There seems to be no way to currently add features such as a Bard's Magical Secrets and a Rogue's choice of Expertise between thieves tools + 1 skill or 2 skills, to a class.

Same with eldritch invocations/pact boons for warlocks, making it impossible to make a custom version using only the editors on the website.