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

frontend/src/utils/date.util.ts

@@ -0,0 +1,26 @@
+import moment from "moment";
+
+export const getExpirationPreview = (
+  name: string,
+  form: {
+    values: {
+      never_expires?: boolean;
+      expiration_num: number;
+      expiration_unit: string;
+    };
+  }
+) => {
+  const value = form.values.never_expires
+    ? "never"
+    : form.values.expiration_num + form.values.expiration_unit;
+  if (value === "never") return `This ${name} will never expire.`;
+
+  const expirationDate = moment()
+    .add(
+      value.split("-")[0],
+      value.split("-")[1] as moment.unitOfTime.DurationConstructor
+    )
+    .toDate();
+
+  return `This ${name} will expire on ${moment(expirationDate).format("LLL")}`;
+};