Grouping and tagging system

[amdw]

It would be useful to be able to group and tag objectives, in order to provide views that aggregate objectives together.

For example, objectives could be grouped into projects or products, so it's possible to see how much of a team's resources are being spent on each, and get a sense of the stack ranking of these aggregates.

Also, tags would be useful to provide highlighting of objectives with certain themes.

Grouping would be a mutually-exclusive concept: each objective could be a member of at most one group of each "group type" (e.g. "project").

Tagging, by contrast, would be a many-to-many concept: each objective could have multiple tags, and each tag could be applied to multiple objectives.

[amdw]

I think we now have an implementation good enough to call "done" for the time being.