web app: on project profile page add graphs for historical progression of coverage per-fuzzer

[DavidKorczynski]

Ref: google/oss-fuzz#11398 (comment)

[phi-go]

I think we can close this issue.

[evverx]

Agreed. I think it can be closed.

I can see spurious coverage drops affecting specific fuzz targets:

With those graphs it should be easier to figure out how stable the data is in the context of using it to file bugs reports automatically. It doesn't look like it's stable enough currently.