Implement a `Camp.year` property

**Is your feature request related to a problem? Please describe.**
In 48 places we get the year of a camp by `Camp.camp.lower.year` which gets the year from the lower part of the camp date range.

**Describe the solution you'd like**
Create a `year` property returning `self.camp.lower.year` and change all 48 places to use this new property.