Feat: Adds support for all functionalities of a Group.

[jawidMuhammadi]

This PR has been created on top of PR #68 that has been developed by Mr. Saksham Handu in GSOC'2019, and this PR adds support for following functionalities and improvements which were missing in his PR:

1. Edit and Update group details
2. Edit or delete group members name
3. Edit or delete group leaders name
4. Reopen a group if it has been closed
5. Activate a group if it is pending
6. Close a group account
7. UI refactoring in GroupdDetailsFragment, AddGroupMemberFragment, AddGroupLeaderFragment, and ReviewFragment to provide a better UX/UI experience to users based on Material Design guidelines.
8. Adds Coroutines Library to call APIs in Background Thread and Handle Errors in a better way which is Google's recommended way.
9. Refactored the signature of the creating group method to handle network errors.

• Viewing Group Details and Group Tasks(Reopen, Close, Activate)

• Edit Group Details:

• Add, Edit, and Delete Leaders Name:

Please make sure these boxes are checked before submitting your pull request - thanks!

• Apply the AndroidStyle.xml style template to your code in Android Studio.

• Run the unit tests with ./gradlew check to make sure you didn't break anything.

• If you have multiple commits please combine them into one commit by squashing them.

[jawidMuhammadi]

Dear @therajanmaurya, @awasum Please take a look at this PR.