Remove unauth GitHub api

[chunweii]

Summary:

Fixes #1220

Changes Made:

• Use the unauthenticated REST API to fetch the settings first, and only fall back to using raw.githubusercontent.com if the REST API fails.
• To test it out you can run the following bash command on your computer to hit the rate limit of 60
  for i in {1..60}; do curl https://api.github.com/repos/CATcher-org/public_data/contents/settings.json; done;

Proposed Commit Message:

Use rest api first, then raw githubusercontent

REST API should be more reliable, but it has a rate limit
for unauthenticated requests. Let's use REST API call for
the fetching of settings.json, with a failsafe being
raw.githubusercontent.com.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e8cab15) 53.95% compared to head (636350b) 53.83%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
- Coverage   53.95%   53.83%   -0.13%     
==========================================
  Files         101      101              
  Lines        2882     2885       +3     
  Branches      534      534              
==========================================
- Hits         1555     1553       -2     
- Misses        985      988       +3     
- Partials      342      344       +2     

Files	Coverage Δ
src/app/core/services/github.service.ts	15.17% <ø> (-0.33%)	⬇️

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[damithc]

@CATcher-org/active-devs shall we get this PR merged quickly, while students are still smoke testing CATcher?

[gycgabriel]

LGTM