(bug 50425) Add the flood flag on bs.wikipedia

Adding the flood flag on bs.wikipedia per community request;
administrators will be able to add and remove users from this
group (including themselves).

[Note: The request asked for bureaucrats to be able to assign
this group to users, too, but since there are no bureaucrats
who are not simultaneously administrators on bs.wikipedia, I'm
adding this right only to the 'sysop' group.]

Bug: 50426
Change-Id: Idb753d1b28826ea492d7d8f3854e76dac9a6e25a