Allow Users to Delete Draft Testimony

## Problem

On MAPLE, we intentionally don't allow users to delete published testimony (under the principle that public testimony is public). That said, we should allow users to delete *drafts* of testimony, which are clearly not intended for public consumption

## Success Criteria
- [ ] Add a button that allows users to delete drafts of testimony
- [ ] Deleted drafts do not count toward the MAX_VERSION limit of testimonies
- [ ] Published testimonies are still never deleted

## Additional Info
- TODO: Design/UX/Figma
- In which of these places should this button live?:
- The Submit Testimony Flow
- The "Draft Testimony" List on the Edit Profile page
- The Testimony Detail page for a submitted testimony (when relevant - this page should only exist for drafts that are edits of existing testimony, not drafts that have never been published).
- Do we need to do anything extra to establish that only *drafts* may be deleted, not *published testimony*? e.g.:
- Only display the delete button for testimony with an active draft (obvious)
- Phrasing of the delete button (e.g. "Delete Draft" or "Discard Draft" instead of "Delete")
- Tooltip on the delete button (e.g. "This will only discard un-published changes - published testimony is permanent")