refresh on pull implemented for group screen tabs + UI changes to the auth screen

[Rushour0]

Fixes #
Added a custom pull to refresh mentioned in the issue #91
I have also made UI changes to the auth screen mentioned in the issue #158

Describe the changes you have made in this PR -
Issue 91
Made a new TabControllerStorage since we cannot late initialise the tabcontroller, and thus cannot declare it out of the build method. Storing the tab index using this singleton implementation.
Added a new package custom_refresh_indicator to implement the pull to refresh feature with a custom icon

Issue 158
Made changes to the UI to make it better looking and user friendly.

Screenshots of the changes (If any) -
Issue 91

pull_to_refresh_feature.mp4

Issue 158

Note: Please check Allow edits from maintainers. if you would like us to assist in the PR.

[Rushour0]

please check the new commit, fixed the getter setter error

[ItsAdityaKSingh]

Hey @Rushour0! Could you make a new branch and rebranch these changes? It's a good practice to create branches for changes you are proposing rather than making a pull request directly from the main branch :)
Also, the buttons' border radius that you have changed makes it a more box-like structure. Let's stick to a more curved button style, so if possible, could you change it too?

[Rushour0]

@ItsAdityaKSingh Alright, I will make the due Button UI changes and push it to a new branch itself
Should I make a PR to development branch or make my own branch and push the whole repo with the branch in it?

Thanks for the help!