It must not be possible to add multiple identical vendors

[mcjaeger]

ehem, looks like we do not have any constraints on vendor creation. Shall be limited fo course. Merge Vendors would be a nice feature also as optional thing in the ticket.

(from tgrafs issue on our internal system) Currently it is possible - by mistake - to enter a vendor multiple times, see for example "npm, Inc". SW360 shall check whether a vendor already exists before adding a new database entry.

(remark: first wrongly entered in the fossology project: fossology/fossology#1006)