Simplify adding paragraph that lists sponsors

[jdleonard]

Following up from #92.

Things would be simpler if:

• "Element - Sponsor" were renamed to "Element - Sponsors"
• The "Sponsor" dropdown for "Element - Sponsor" were removed (it should always be set to "Sponsors")
• There were no "Argument" field (the tier is already specified by selecting the correct block)

None of these things really matter for us, but we're looking to share the codebase with other camps so it would be nice to simplify.