Foundation model changes for meeting minutes

[thibaudcolas]

Related: #1464. I’d like to make the following changes to Meeting-related models to make my secretary job easier:

• ApprovedGrant entity: support reStructuredText so we can have links
• ApprovedCorporateMember: add a CharField with choices for sponsorship levels (check best field name based on related code)
• ActionItem task: support reStructuredText so we can have links

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I can’t manage assignees here but I’d like to work on this myself.