Enforce explicit function return types

This re-enables the `@typescript-eslint/explicit-module-boundary-types`
check and also enables the
`@typescript-eslint/explicit-function-return-type` check.

Signed-off-by: David Mehren <git@herrmehren.de>

src/users/users.service.ts

@@ -26,7 +26,7 @@ export class UsersService {
     return this.userRepository.save(user);
   }
 
-  async deleteUser(userName: string) {
+  async deleteUser(userName: string): Promise<void> {
     // TODO: Handle owned notes and edits
     const user = await this.userRepository.findOne({
       where: { userName: userName },