FEATURE: When creating the spreadsheet, only update if it has changed

[pconrad]

When creating the spreadsheet with members, first:

• See if there is an existing spreadsheet
• If so, check the contents of each of the tabs
• If the team membership matches any of the existing tabs, don't update it.
• Otherwise, create a new tab

Optional but nice to have:

• The new (latest) tab should be always be called "Members" or perhaps "Members_yyyy-mm-dd-hh-mm"
• If the new membership is different from all existing tabs, rename all existing tabs that don't start with "OLD_" to start with "OLD_"
• Number the students 1 through 4.