Add Admin Posting Details Page

[albertlai431]

Ticket link

Closes #155

Implementation description

• Refactored VolunteerPostingDetails page
• Created routing for AdminPostingDetails page
• Created query to fetch Posting data and the associated Admin Posting Details Page UI

Steps to test

1. docker-compose up --build
2. Navigate to http://localhost:3000/admin/posting/1 and verify that the UI matches that specified in Posting details page for schedule creation #155

What should reviewers focus on?

• Verify that the query works and the UI matches that specified in Posting details page for schedule creation #155

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

[github-actions]

Visit the preview URL for this PR (updated for commit 93bb27c):

https://sistering-dev--pr167-albert-posting-detai-3x69ep8l.web.app

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[LenaNguyen]

Just one small thing!

[LenaNguyen]

[sherryhli]

Awesome, LGTM!