feat: reverse shares (#86)

* add first concept

* add reverse share funcionality to frontend

* allow creator to limit share expiration

* moved reverse share in seperate module

* add table to manage reverse shares

* delete complete share if reverse share was deleted

* optimize function names

* add db migration

* enable reverse share email notifications

* fix config variable descriptions

* fix migration for new installations

backend/package.json

@@ -67,7 +67,7 @@
     "eslint-plugin-prettier": "^4.2.1",
     "newman": "^5.3.2",
     "prettier": "^2.8.2",
-    "prisma": "^4.8.1",
+    "prisma": "^4.9.0",
     "source-map-support": "^0.5.21",
     "ts-loader": "^9.4.2",
     "tsconfig-paths": "4.1.2",