feat: add instagram_url in settings model

[snitin315]

Short description of what this resolves:

Refers fossasia/open-event-frontend#4617

Changes proposed in this pull request:

Added field for an Instagram link in admin settings.

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[codecov]

Codecov Report

Merging #7163 into development will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           development    #7163   +/-   ##
============================================
  Coverage        62.87%   62.87%           
============================================
  Files              259      259           
  Lines            12990    12992    +2     
============================================
+ Hits              8167     8169    +2     
  Misses            4823     4823           

Impacted Files	Coverage Δ
app/api/schema/settings.py	100.00% <100.00%> (ø)
app/models/setting.py	86.20% <100.00%> (+0.11%)	⬆️

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