Add spinner to indicate app loading

[sumitde22]

Description

Application now shows a loading spinner instead of flashing the no courses page temporarily when a user logs in

Makes login look a little cleaner

Closes #828

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe how you tested this PR (both manually and with tests)
Provide instructions so we can reproduce.

• Test A: Logged in with student account in dev and loading spinner showed before opening homepage
• Test B: Logged in with student account with no courses in dev and loading spinner showed before showing no courses page

Checklist:

• I have performed a self-review of my own code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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🔍 Inspect: https://vercel.com/sandboxneu/office-hours/8SWWkp3w7TkEBALXgnNDcyqLtbNu
✅ Preview: https://office-hours-git-nocoursesflash-sandboxneu.vercel.app

[sumitde22]

Small note: I realized after pushing that I unnecessarily renamed the didRedirect hook and I made another commit to revert that, but it's not showing up in the PR

[ashdawngary]

looks good! (checked on local)