Add last_updated_by Foreign Key To Grades and Marks Tables

[lizzie-liu]

Proposed Changes

This PR adds a last_updated_by foreign key column (referencing roles.id) to both the marks and grades tables. This allows instructors to track which TA (or instructor) last assigned a given grade.

Changes:

• Database migration adding last_updated_by column, index, and FK constraint to both tables
  • Column is nullable with a default of null
• Mark and Grade models updated with belongs_to :last_updated_by association
• Role model updated with inverse has_many associations for last_updated_marks and last_updated_grades with dependent: :nullify
• Tests added for new associations in mark_spec.rb, grade_spec.rb, and role_spec.rb

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[coveralls]

Pull Request Test Coverage Report for Build 23692078518

Details

• 8 of 8 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 91.713%

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Totals
Change from base Build 23690924504:	0.001%
Covered Lines:	45168
Relevant Lines:	48441

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💛 - Coveralls

[david-yz-liu]

@lizzie-liu this is good, but please also define the "inverse" associations within the Role class (a has_many)