samplchallenges/SAMPL8: SAMPL8 CB8 "drugs of abuse" challenge inputs

This archives the final inputs that were available for the SAMPL8 CB8 drugs of abuse challenge. Previous changes included here were:

• June 9, 2020: Add first info on CB8 "drugs of abuse" challenge from Isaacs.
• June 16, 2020: Fix cocaine structure, which was incorrectly listed as the ethyl ether rather than the methyl ether. Affects all G7 files.
• August 21, 2020: Add CB8 submission template file and host-guest instructions (for now only for CB8, other host-guest system information will be updated as it becomes available).
• Sept. 2, 2020: Add submission instructions. The submissions system is now live at http://sampl-submit.us-west-1.elasticbeanstalk.com/submit/SAMPL8-Isaacs.