Creates/modifies/deletes a chain.
Usage: !chain chainname site1 [site2 [site3] ...]
If a site is not found in the chain, the site is being added.
If the site is found in the chain, the site is being removed.
If a chain does not exist, it is being created.
If a chain gets empty, the chain is removed.
Examples:
<cmd> GRP HQ1 HQ2 <- chain GRP is created, HQ1, HQ2 are added.
<cmd> GRP HQ2 HQ3 <- HQ2 is removed, HQ3 is added (so chain GRP has HQ1 and HQ3 now)
<cmd> GRP HQ1 HQ3 <- chain GRP is removed.