Add base API class
What does this MR do?
This adds a base API class and autocorrects all existing API resources to use it. Mainly for gitlab-com/gl-infra/scalability#462 (closed), but may help with #215230 too.
Is using a base class a good idea? I don't know! https://github.com/ruby-grape/grape/issues/1431#issuecomment-396167355 seems relevant, but I tested a similar case to what we want this for in https://gitlab.com/smcgivern/grape-class-method-test and it seems to work OK.
Edited by Sean McGivern