Gradebook: ‘Excuse/Include Grade’ with percentage entry distorts scores

[hornersa]

jira: https://sakaiproject.atlassian.net/browse/SAK-49811

Much of the logic added to saveExclude is adapted from the sibling methd, saveGrade. I also removed a redundant block of calls from the saveGrade method; the three deleted lines are a repetition of three lines directly above.

[hornersa]

I'll apply the above suggestions and force push a new PR.