Add project and branch column to the workitems table

According to Toni

you will see for example that in qtsvg task there are also qtbase workitems. That is the usual case. It is also possible that same workitems are for different branch but same project, happens sometimes during branching when content is same. I think those need to be recorded separetely for each.

So it's not enough to only relate the task to to project and branch. Each workitem needs to be related separately.

Additionally:

the same workitem_id might be found for a build, for example of qtbase-dev and qtbase-6.5. Then what will the API return as "branch" when querying it? That would actually be just the string of first "branch" which actually created the workitem.