Fix the "x of y" displayed at the top of Issuables' sidebar
- We now display the index of the current issuable among all its project's issuables, of the same type and with the same state.
- Also, refactored a bit the Issuable helpers into a new
IssuablesHelper
module. - 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 (closed).