Implement user invitation functionality for projects via OSM messages

[Anuj-Gupta4]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Related Issue

• Related to: Backend: Inviting Users through username or email #1199

Describe this PR

Added an endpoint that will take a project id and username, check if an osm user with such username exists and then sends them an invite with a link to the project.

Checklist before requesting a review

• 📖 Read the FMTM Contributing Guide: https://github.com/hotosm/fmtm/blob/main/CONTRIBUTING.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.

[optional] What gif best describes this PR or how it makes you feel?

[spwoodcock]

Perfect 👍

What I'm thinking for the future to make this flow better for the org admin invite too:

• A new db table for invites, including invite tokens, and selected permissions
• We have a new endpoint to accept invites
• We create tokens (with an expiry) to access this endpoint
• The OSM endpoint includes a link that calls this API (which includes the token)
• Calling the endpoint with the specific token first logs in the user, so the db has an entry for them
• Then we add the specified user roles to the db too (org or project permissions)

Open to other ideas though!

[Anuj-Gupta4]

This sounds like a great idea.