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/api/private/tokens/tokens.controller.ts

@@ -53,7 +53,7 @@ export class TokensController {
 
   @Delete('/:keyId')
   @HttpCode(204)
-  async deleteToken(@Param('keyId') keyId: string) {
+  async deleteToken(@Param('keyId') keyId: string): Promise<void> {
     return await this.authService.removeToken(keyId);
   }
 }