Add mobile post card border

[joannacheng21]

Add mobile post card border

Issue Number(s): #257

Adds border for mobile feed post cards

Checklist

• [x] Requirements have been implemented
• [x] Acceptance criteria are met
• [x] Database schema docs have been updated or are not necessary
• [x] Code follows design and style guidelines
• [x] Commits follow guidelines (concise, squashed, etc)
• [x] Relevant reviewers (Senior Dev/EM/Designers) have been assigned to this PR

Critical Changes

None

Related PRs

None

Testing

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Coverage report for web

St.:grey_question:	Category	Percentage	Covered / Total
🔴	Statements	26%	526/2023
🔴	Branches	9.3%	64/688
🔴	Functions	12.42%	77/620
🔴	Lines	22.33%	401/1796

Test suite run success

40 tests passing in 5 suites.

Report generated by 🧪jest coverage report action from 0b775ed5f7a53bb2357491950f15be1ad4137571