[Fix #4673] Migrate EvalAI CI/CD Workflow to Github Actions

[Zahed-Riyaz]

As a fix for Issue #4673 , his PR aims to :

• Completely migrate EvalAI's current CI/CD workflow from Travis CI to Github Actions.
• Setup CI checks for linting, migrations, and static type checks.
• Decommission .travis.yml and remove Travis-specific config/integration from the repository.
• Update documentation to reflect the CI migration : make changes in README.md and other files to reflect the changes made.

The github actions default variables to replace the travis CI variables are provided this link : https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/store-information-in-variables#default-environment-variables

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.67%. Comparing base (96968d6) to head (d677b49).
Report is 1188 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4685      +/-   ##
==========================================
- Coverage   72.93%   69.67%   -3.26%     
==========================================
  Files          83       21      -62     
  Lines        5368     3608    -1760     
==========================================
- Hits         3915     2514    -1401     
+ Misses       1453     1094     -359     

see 73 files with indirect coverage changes

see 73 files with indirect coverage changes

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[Zahed-Riyaz]

Hi @RishabhJain2018 this PR is ready for review.
I've updated all the travis variables to github actions variables
-TRAVIS_BRANCH <->GITHUB_REF_NAME
-TRAVIS_PULL_REQUEST <-> checks to see if GITHUB_EVENT_NAME == "pull request" & so on.
-All the Travis CI checks are passed by Github actions.
-Package and Deploy is for main/master branch only, so it skips.

The docs have also been been updated to reflect Github Actions now instead of Travis CI. Let me know if any changes are to be made.