Change how target tags are retrieved in the UI

[david-wu-octopus]

We are changing how target tags (formerly roles) are stored behind the scenes. Previously, the list of roles would be constructed by examining all available deployment targets and finding all the distinct roles. Now the list of roles/tags is retrieved directly from its own purposely-built table.

The UI will use the new deploymenttargettags endpoint that implements this functionality. This will replace usages of the existing machineroles/all endpoint.

This change will improve performance and user experience. In particular, target tags created in the process editor will be immediately available when creating a deployment target and vice versa.