Feed UI implementation

Implement all Feed UI components (#118)

* Homepage story and schedule UI implementation.

* Fixed tests for feed viewmodel.

Refer feed_viewmodel_test.dart for examples on how to write unit tests
for the functions that use Task to call functions from the API service.
Added test for failure case in stories API call.

* Implemet MM articles UI and improve test coverage.

This commit adds UI implementation for MM articles, changes the story
component widget to match the design spec, and adds tests for Margin,
Failure. Unused code and tests related to API classes have been removed.

* Feed view implemented.

Feed view UI components implemented:
[x] Stories
[x] Schedule
[x] Stories this week
[x] Upcoming Events

Internet permission added to AndroidManifest.xml, Story component has a
border gradient now. Files for exporting the files in some folders are
added to make documentation generation work correctly.

* Implement suggested changes.

- Deleted test/.test_coverage.dart
- Use Future.wait() in FeedViewmodel.initialise().