Persist weekday and time slot info in plan records

[timwangmusic]

Description

Plan records do not have time related info. This leads to confusion when front-end needs to post those data in order to have a complete view of the plan.

Solution

• Weekdays are saved as their English names in the records.
• Time slots are saved with a format from x to y.

Example Record

...
"weekdays":["Saturday","Saturday","Saturday","Saturday"],
"time_slots":["from 10 to 12","from 12 to 13","from 14 to 17","from 18 to 20"]

Testing

• [ ] Integration testing on Heroku staging
• [x] Added new unit tests

Checks

• [x] Have you removed commented code?
• [x] Have you used gofmt to format your code?