Refactor admin create posting pages into single page component

[7tint]

Ticket link

Closes #213

Implementation description

• Combined steps of the admin create posting process into a single page component
  • Instead of the three steps (basic info, shifts, review) having different URLs, all steps are now at http://localhost:3000/admin/posting/create
• Fixed an incorrect query I noticed on the basic info page

Steps to test

1. Navigate to http://localhost:3000/admin/posting/create and try creating a posting. This should still work as before.

What should reviewers focus on?

• Didn't break anything 😓

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

[7tint]

@MatoPlus I left the three pages (CreatePostingBasicInfoPage, CreatePostingShiftsPage, CreatePostingReviewPage) where they were in src/components/pages/admin/posting.

But they are technically not "pages" with individual routes anymore (since they all live under the new CreatePostingPage).

Let me know if you want this moved elsewhere.

[github-actions]

Visit the preview URL for this PR (updated for commit 1151e8e):

https://sistering-dev--pr496-refactor-admin-posti-a4byk68d.web.app

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[neelismail01]

Everything is still working on my end 😁

Feel free to merge when you get a chance.