Implement graphql mutation node for project stats summary

[susilnem]

Addresses

• https://github.com/the-deep/server/issues/1438

This PR doesn't introduce any:

• [x] temporary files, auto-generated files or secret keys
• [x] n+1 queries
• [x] flake8 issues
• [x] print
• [x] typos
• [x] unwanted comments

This PR contains valid:

• [ ] tests
• [ ] permission checks (tests here too)
• [ ] translations

[k9845]

@susilnem Make sure you don't resolve the conversation, reviewer with resolve once fixed.