Allow multiple settings in same guild.

[aryanc403]

This can be done by adding a unique id for each reminder settings.

[aryanc403]

Also in this case default for all websites should be None and one should opt for websites with each settings.

[prabowo02]

I have created changes that have this similar behaviour in my own branch
But I don't think this worth a PR as I was just making the minimal changes to cope with my need, so I will just put it here for your own reference.

In short, in my own branch, each guild can have multiple setting names. Most commands now must have a <setting_name> argument. For example t;remind settings setting_name_1 or t;remind subscribe setting_name_2 codeforces.com