Add generations admin support

[jzheaux]

Currently, the Spring Website API can GET a list of support generations.

It would be nice to be able to use it to add generations as well, similarly to how it works with releases.

The reason this would be nice is to allow for using Postman for something that otherwise requires SSOing in, navigating, and editing in a UI for what amounts to a very small change.

What I'd like to be able to do is:

POST /generations

{ "name" : "6.1.x" }

And this would create a generation with the initialReleaseDate of today and the appropriate ossSupportEndDate and commericalSupportEndDate accordingly.

This is also quite nice for the purposes of automation; any time Spring Security releases a new minor version, it could invoke the POST /generations endpoint to automatically add it to the support page.