Redirect to 404 page when posting does not exist

[briantu]

Ticket link

Closes #144

Implementation description

• Added a /not-found route for NotFound.tsx component
• In VolunteerPostingDetails.tsx, redirect to /not-found when posting does not exist
  • Note: I had to change the way the postingDetails variable was being set when calling useQuery, because before, loading and postingDetails could be false and null at the same time even though the posting exists

Steps to test

1. Go to /volunteer/posting/{id} and replace id with a posting id you have created in your local database
2. Check that the page still loads when the posting exists
3. Try changing the id to one that does not exist and check that the page redirects to not found

What should reviewers focus on?

• Code correctness

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[briantu]

Oops, I think I have commits from my last PR because I didn't rebase 😅

[github-actions]

Visit the preview URL for this PR (updated for commit 9c8d75c):

https://sistering-dev--pr186-brian-posting-redire-ey6pjjm4.web.app

_{(expires Fri, 11 Mar 2022 03:41:11 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

[LenaNguyen]

Nice use of useQuery !