Allow admins to edit school and major of users

[damianhxy]

Description

• Disable school and major fields on the edit user form for non-admins
• Allow admins to update school and major fields

Motivation and Context

Currently, the school and major fields on the edit user form appear editable, but in reality can be edited neither by users nor admins.

This PR allows admins to update the fields. However, in the interest of preserving integrity of the data, we disable the fields for users.

Fixes #1659

How Has This Been Tested?

• As an admin, update school and major fields successfully for yourself and other users
• As a normal user, fields are disabled for you, and you can't update them (even if you removed the disabled attribute)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting

[20wildmanj]

Verified admins are now able to edit school and major of users, and students have the fields now greyed out. LGTM