Rename Workshift Models

[naderm]

Currently a lot of our models in the workshift models have somewhat unintuitive names and and many of their fields could benefit from being modified. I'd rather not make these changes in the middle of the semester, but I'd still like to keep an open track of what changes I'd like to make to the models in the future:

• Rename ShiftLogEntry to InstanceLogEntry
• Rename RegularWorkshift to RepeatedWorkshift
• Remove WorkshiftInstance.logs, replace with InstanceLogEntry.instance (Turning a ManyToMany relation into a ManyToOne relation)
• Remove WorkshiftProfile.pool_hours, replace with PoolHours.profile (Turning a ManyToMany relation into a ManyToOne relation)