added "Favourites" section and fixed the issue 319

[jaivsh]

Description

For #320, I've added te favourites section where the user can see their favourite or most important exams/assessment details. The implementation is such that it would not require internet data or any additional user model, and extracts favourited assessments/exams right from the "Alerts" modal class.

Sample video of the implementation: See video of favourites section

For #319,

I've fixed the error in signup_forms_additional_details.dart file. This makes the application's user sign up process exception free.

Before:

After:

Fixes #319, Fixes #320

Have you read the Contributing Guidelines on Pull Requests?

• [x] Yes
• [ ] No

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines(Clean Code) of this project
• [x] I have performed a self-review of my own code
• [x] My changes generate no new warnings
• [x] I have added tests/screenshots(if any) that prove my fix is effective or that my feature works.

[jaivsh]

Hi @avinashkranjan can you please review this PR? Thank you!