fix(web): update post perms check to match api server

[CaramelKat]

Resolves N/A

Changes:

Updated the web post permissions check to match the api server to prevent unauthorized posts in communities

• I have read and agreed to the Code of Conduct.
• I have read and complied with the contributing guidelines.
• What I'm implementing was an approved issue.
• I have tested all of my changes.

[ashquarky]

I honestly can't tell the difference between these, but Dani has said the two differ and I believe him ^^
We should probably refactor this out into something cleaner. Moving this endpoint to miiverse-api (via gRPC) might be a good opportunity to draft that.