Revise summary message of sast

Problem

Sast summary message may not be clear enough

Proposal

proposal from @bikebilly at https://gitlab.com/gitlab-org/gitlab-ee/issues/3995#note_56971283

One thing that I feel we are still missing is that if you say "SAST detected 4 vulnerabilities and 2 fixed vulnerabilities", it is not possible to know how many vulnerabilities are introduced by that specific MR (added count), and this should be the first information available. Also, mixing a general information and a diff information in the same sentence adds more confusion, even because expanding the item will show less issues than the count.

What about having the sentence like: "SAST detected 3 new (of 43 total) and 2 fixed vulnerabilities" using counter from added, all and fixed? Sounds more consistent with the list you'll see when expanding the summary.

This also makes really clearer that if you see "0 new (of 43 total)" then the expansion brings you directly to the full report, if no added or fixed items are present.

Difficulties

How does this work in conjunction with the changes going in from https://gitlab.com/gitlab-org/gitlab-ee/issues/4310

Design decision

TODO