Add referred_to_external to PatientConsultation

[Ashesh3]

Adds referred_to_external to patient_consultation to support setting the "Referred To" facility to have an arbitrary value.
Constraints are updated accordingly. Only one of referred_to and referred_to_facility can be set at a time, other one is cleared automatically.

Required for issue: coronasafe/care_fe#4876

@coronasafe/code-reviewers

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.1% Duplication

[codecov-commenter]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.03 ⚠️

Comparison is base (b3839b5) 56.22% compared to head (6a520f8) 56.20%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1228      +/-   ##
==========================================
- Coverage   56.22%   56.20%   -0.03%     
==========================================
  Files         193      193              
  Lines        9489     9498       +9     
  Branches     1599     1603       +4     
==========================================
+ Hits         5335     5338       +3     
- Misses       4100     4105       +5     
- Partials       54       55       +1     

Impacted Files	Coverage Δ
...e/facility/api/serializers/patient_consultation.py	32.62% <12.50%> (-0.53%)	⬇️
config/urls.py	77.77% <66.66%> (-2.23%)	⬇️
care/facility/models/patient_consultation.py	95.65% <100.00%> (+0.06%)	⬆️

... and 1 file with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.