Convert 'submissionGuideline' to 'body' for rfp content

[ilikesymmetry]

What's included in this pr + description

• copy data from submissionGuideline to body in RfpMetadata's content
• have application rely on body

Note: removing submissionGuideline in a followup PR to keep inline with best practice of (1) new data & logic, then (2) remove old unused data & logic

Motivation & Context

• The purpose of an RFPs body is to describe it's purpose and guide proposals, serving the same utility as "Submission guidelines". They should be one data field in reality.
• Doing this now to before self-serve RFP to not produce more data & logic that will need to be overridden

Steps to test

1. Go to an RFP, observe how submission guidelines are still intact
2. Propose to the RFP, notice how submission guidelines are still intact
3. Everything should be the exact same and the change unnoticeable

[linear[bot]]

WEB-993 Convert content.submissionGuideline storage to content.body

[vercel[bot]]

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