Bot doesn't check if arguments are specified

E.g. !alias-del will crash the bot

Oct 07 07:35:33 matrix-bot python3[6500]:   File "/opt/jonathan/jonathan/__main__.py", line 149, in on_message
Oct 07 07:35:33 matrix-bot python3[6500]:     await on_admin_message(room, event)
Oct 07 07:35:33 matrix-bot python3[6500]:   File "/opt/jonathan/jonathan/__main__.py", line 312, in on_admin_message
Oct 07 07:35:33 matrix-bot python3[6500]:     alias = part[1]
Oct 07 07:35:33 matrix-bot python3[6500]:             ~~~~^^^
Oct 07 07:35:33 matrix-bot python3[6500]: IndexError: list index out of range

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