API：評論時間改成unix timestamp

[anntsai2356]

使用者的評論時間改成 unix timestamp 且只顯示 2019/01/01

[lukechu1997]

make a migration file for data type changes timestamp -> int(?
set input time as Unix time in the models
https://stackoverflow.com/questions/35915576/laravel-timestamp-saving-time-as-unix-timestamp