UnyCloud v0.18.4

Changes in UnyCloud v0.18.4:

✅Changed README.md
✅Changed RELEASE_NOTES.md
✅Changed UNYCLOUD_VERSION
✅Changed docs/MAINTENANCE.md
✅Changed docs/installation.md
✅Changed frontend/package.json
✅Changed frontend/public/pwa-install.js

Commit: e88fa9d