Assign categories to expenses (#28)

* add expense categories

* set category to Payment for reimbursements

* Insert categories as part of the migration

* Display category groups

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Co-authored-by: Sebastien Castiel <sebastien@castiel.me>

prisma/schema.prisma

@@ -29,11 +29,20 @@ model Participant {
   expensesPaidFor ExpensePaidFor[]
 }
 
+model Category {
+  id       Int       @id @default(autoincrement())
+  grouping String
+  name     String
+  Expense  Expense[]
+}
+
 model Expense {
   id              String           @id
   group           Group            @relation(fields: [groupId], references: [id], onDelete: Cascade)
-  expenseDate     DateTime         @db.Date @default(dbgenerated("CURRENT_DATE"))
+  expenseDate     DateTime         @default(dbgenerated("CURRENT_DATE")) @db.Date
   title           String
+  category        Category?        @relation(fields: [categoryId], references: [id])
+  categoryId      Int              @default(0)
   amount          Int
   paidBy          Participant      @relation(fields: [paidById], references: [id], onDelete: Cascade)
   paidById        String