Revamp the Row Level Security template

[RogerTangos]

The Row Level Security policies template could really use some love.

It should ideally

• Pre-populate the username field
• Explain how row level security keeps users data secure
• Explain how to use the RLS policy grant language to create policies (GRANT [access type] ACCESS TO [user] ON [table name] WHERE [policy] )
• Allow a user to preview what their grantee will see when they access the table