Create aliases

[rubenvanerk]

When merging athletes, only one athlete is saved and the rest is deleted. This is problematic when a new competition is imported with the name of one of the deleted athletes. The parser will create a new athlete and they have to be merged again.

• Athletes should be kept but an extra column should be added to point to the actual athlete: alias_of.
• A new manager should be created that filters out athletes that are an alias of another athlete.
• Alias athlete should redirect to main athlete.