[#12894] Calculate msq totals by student email instead of name

[itstrueitstrueitsrealitsreal]

Fixes TEAMMATES#12894

Outline of Solution
Following the discussion in TEAMMATES#12894, I modified both mcq-question-statistics.component.ts and msq-question-statistics.component.ts to take recipient emails into account.

I also updated the code in msq-question-statistics.component.ts to be more in line with the code in mcq-question-statistics.component.ts, calling toFixed(2) to format the values in the table.

In addition, I changed the tests to reflect the change from indexing students by name to indexing them by email.

I'd like to credit the authors of TEAMMATES#12981, as I used their work to formulate my solution.

[github-actions]

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