MVVM implementation for Add Event Page (add_event_page.dart).

[rutvik11062000]

Is your feature request related to a problem? Please describe.
This is the child issue of #758
To implement MVVM architecture on add_event_page.dart file.

Describe the solution you'd like
See #760 for in-depth details about the implementation.
For Sample see the PR changes #759

Additional context
Other things to take into consideration:

1. Make sure all the files that you are either updating or creating must be under 300 lines.
2. If there are multiple same widget with only few modification extract it as a method and give only change variable as a parameter.
3. Make the code as concise as possible.
4. Kindly add setup locator in the add_post_test.dart otherwise travis build will fail.

[CoderMayhem]

This page is built on top of events page having common endpoints and functions. I guess this can be handled along with the events.dart page. I guess you can close this issue and update it in the issue description in events page MVVM implementation issue. @rutvik11062000