exclude Yourself player from Tournament set

[abagali1]

cleaning up my fork - copied from previous PR

• change include_users, bye_player, and add_users form field querysets from Submission.objects.latest() to Submission.objects.latest().exclude(user=get_user_model().objects.get(username="Yourself"))
• remove ValidationErrors regarding inclusion of "Yourself" Submission as a Tournament player

[coveralls]

Coverage decreased (-0.04%) to 33.52% when pulling 07b53a4 on abagali1:exclude-yourself into 966f92b on tjcsl:master.

[abagali1]

CI was failing for a somewhat legitimate reason:

Referencing the User model, specifically to access the "Yourself" player, created a circular dependency which caused the initial db migration to fail (the "Yourself" player is created in a db migration but referencing "Yourself" in TournamentCreateForm before that migration is applied caused the CI fail since the CI reapplies all migrations every run)

This change worked locally since my local environment had already applied the necessary migrations.

I modified the code to achieve the same goal but this method is considerably jank and this change is not necessarily required because the form validator prohibits the "Yourself" player from being included in the set. This is purely a frontend clarification