As a dashboard user, I can add a stage to a release pipeline

[matthewelwell]

Users can add stages to a given release pipeline. When creating a stage, the user is required to provide the following information:

1. Name
2. Environment (from within the current project)

Note that the environment can be different between different stages in a given pipelines.

We should impose a limit on the number of stages in a given pipeline. This limit is not really a technical challenge, but more to prevent users from shooting themselves in the foot. Suggest that, to start with, we set this limit at 30 stages.