Fix #809: Introduced StoryThumbnail and updated CompletedStoryList model

[rt4914]

Explanation

Changes

• Introduced StoryThumbnail in StorySummary and also updates the implementation of PromotedStory by using the StoryThumbnail.
• Created a new model CompletedStory because we need topicName also in CompletedStoryList
• Removed some unused models like TopicProgressList and StoryProgressList and therefore code gets updated.

NOTE - This PR has domain and app module code because of we fix only domain layer code then the develop will have crashes.

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The PR follows the style guide.
• The PR does not contain any unnecessary auto-generated code from Android Studio.
• The PR is made from a branch that's not called "develop".
• The PR is made from a branch that is up-to-date with "develop".
• The PR's branch is based on "develop" and not on any other branch.
• The PR is assigned to an appropriate reviewer in both the Assignees and the Reviewers sections.

[veena14cs]

LGTM.