Only show volunteer posting details if user in branch

[briantu]

Ticket link

Closes #612

Implementation description

• If volunteer is not in posting branch, then redirect to not found page

Steps to test

1. Log in as volunteer
2. Try to see posting details for branch current volunteer is not in
3. Check you are redirected to not found page

What should reviewers focus on?

• Code correctness

Checklist

• [x] My PR name is descriptive and in imperative tense
• [x] 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
• [x] I have run the appropriate linter(s)
• [x] 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]

This fix works, but not sure if it's the best way to do it cuz it's a bit messy. Lmk if this is okay - if it is, then I'll also do it for the volunteer posting availabilities page!

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Visit the preview URL for this PR (updated for commit 8e566d0):

https://sistering-dev--pr637-brian-only-show-post-6joolkii.web.app

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[briantu]

hmm would it make more sense to do this on the server side instead? ie: for the posting query, if the current user is a volunteer, make sure the posting is in the same branch as the volunteer, otherwise, on error we redirect instead of showing a toast @briantu would that be better?

Wait yeah that sounds like a better solution