Allow edits of custom cohorts

[mitalia]

When a user edits a cohort they have permission to see, they should be able to add or remove samples. After edits are complete, the system should version the cohort so that membership can be controlled. These versions should be rendered as part of the cohort name whenever they are displayed in the UI for either query or in the variant details view. I suggest a simple numbering scheme where edits cause the cohort to increment by 1 over time. Alternatively, we could use a date stamp, but this is bulky.

Permissions get a little tricky. To simplify matters, I suggest using the following rules:

For project cohorts, the cohort can only contain samples from a single project. Users should pick the project to associate the cohort with, and only be allowed to select samples for that project.

For global cohorts, admins can add any arbitrary samples no matter their project (e.g. admins can tweak the "World" cohort or perhaps a custom ethnicity cohort)