Added new button to launch the new
stage form for customisable cycle
analytics.

Move add stage button to vue component

Added basic test for add stage button

Move css to bootstrap classes

Removes scss styling that can be applied
via bootstrap classes and adds the relevant
classes to html elements

Added feature tests for add stage button

Moved add stage functionality to ee mixin

Created addStageMixin for the add stage
functionality needed in EE. Added a stub
mixin to the CE dir.