Create frontend utilities using Moment.js

[Boyazhang1]

Ticket link

Closes #145

Implementation description

• created 3 moment helper functions in the file DateTimeUtils.ts

• formatTimeHourMinutes and getElapsedHours have been implemented in VolunteerAvailabilityTableRow.tsx to display the time as (5:00pm) and calculate the difference in hours between two given times

• 

• formatTimeHourMinutes can also be used to display the start and end times for events in PostingDetails.tsx and PostingCard.tsx

• formatDateStringYear have been implemented in PostingDetails.tsx and PostingCard.tsx to display the start and end dates in the format (Monday, Feb 20, 2021)

• Looking at the Figma, it can also be used in "Requests Pending Confirmation"

Steps to test

Navigate to http://localhost:5000/graphql and run the mutations (see: #140)

Navigate to http://localhost:3000/volunteer/postings to check if formatDateStringYear formats the dates correctly
Navigate to http://localhost:3000/volunteer/posting/1 to check if formatDateStringYear formats the dates correctly
To test formatTimeHourMinutes and getElapsedHours, the VolunteerAvailabilityTableRow component must be added to an existing page

What should reviewers focus on?

1. If the current implementations are appropriate
2. If there are more places to use them
3. If other moment.js utilities need to be made

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

[github-actions]

Visit the preview URL for this PR (updated for commit 37fd876):

https://sistering-dev--pr187-boya-create-datetime-6talozbe.web.app

_{(expires Mon, 14 Mar 2022 02:40:37 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

[sherryhli]

Nice work! Let's handle the precision of getElapsedHours as described in the attached design notes and then this is good to go

[sherryhli]

Small typo to fix

[sherryhli]

LGTM! Please address the lint warning prior to merging if you can though

[sherryhli]

LGTM 🚢