Send notification message to new project manager upon assignment and refactor assignment code

[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

• Resolves: Assign project admin during project creation #1573

Describe this PR

Refactored add manager endpoint to only allow organisation admins to assign managers
added code to send osm message and email to the new manager upon assignment

Screenshots

Review Guide

Verify project url would work on production.

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?