database: change datetime into start and end time strings

[abhiramtilakiiit]

Overview

The current way of storing DateTime elements in the database is causing database corruption.
Storing strings in the datetime period feels more safe and also prevents other timezone problems like #48
from happening.

Changes Proposed

1. Store times as string without any timezone data, assuming everything as IST.
2. Store timings of event as two variables, rather than a tuple, to prevent any unwanted confusion in db
3. Store duration of the event for security for now. It will not be used currently anywhere, except if needed to restore corruption in db in times of the events

Linked PRs

• Events: split datetimes and add durations events#29

[bhavberi]

Add the timezone message/information in the Event Form (so that user knows in what timezone they should select the time)