Check for self-summoning

[sehaswell]

Would be good to check if a comment/update that has triggered the bot was done by the bot user to reduce risk of infinite comment loops.
e.g. if a user contributing to an MR was called @gitlab-cla-bot check then the post-check comment made by the bot after the first check (i.e. MR creation) would include that text and thus trigger itself, ad infinitum.