Standardize when a membership starts and ends

[oleast]

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Memberships base themselves on starting on July 1st of a year, and ending on Semptember 15th of a year. This has not been standardized in a good way in the code, and can lead to problems where we don't extend memberships correctly.

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I would like the code to have a good way to get what time of a year the a membership/semester starts and ends without specifying if all over the code.

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Rewriting memberships completely?

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#1399