feat: sbml export

[martinjrobins]

• adds read-only field to combined_model serializer that gives the model as sbml
• adds button to model tab that displays the sbml in a dialog (similar to existing mmt display)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.70%. Comparing base (112f27e) to head (ea5ec50).
Report is 30 commits behind head on ui-enhancements.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           ui-enhancements     #646      +/-   ##
===================================================
- Coverage            76.70%   76.70%   -0.01%     
===================================================
  Files                  117      117              
  Lines                 5705     5717      +12     
===================================================
+ Hits                  4376     4385       +9     
- Misses                1329     1332       +3     

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[sonarqubecloud]

Quality Gate passed for 'pkpdapp'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate failed for 'pkpdapp-team_pkpdapp_frontend'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud