Fix the "x of y" displayed at the top of Issuables' sidebar

1. We now display the index of the current issuable among all its project's
issuables, of the same type and with the same state.
1. Also, refactored a bit the Issuable helpers into a new `IssuablesHelper`
module.
1. Added acceptance specs for the sidebar counter.

Note: I didn't add a CHANGELOG item since it's a bug fix for an unreleased version.

Fixes #13356.

See merge request !2818