Subcommunities Milestone 2 fixes

[alejandromumo]

Tasks

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1. subcommunities: request form text changes #909
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2. subcommunities: request page text changes #910
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3. subcommunities: projects search not always showing results #906
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4. subcommunities: display validation errors in form #898
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5. Make 'visibility' settings part of the payload and process it in the backend
6. Backend: pass community to schema #901
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7. Backend: service layer allows only communities that allow children #902
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8. New community request shows up as "deleted"
9. UI: autofill community title and slug after project selection #903
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10. subcommunities: redirection url after request is created has to be to parent community #904
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11. UI: fix request page back button #905
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12. subcommunities: On the freshly created request, add a "System" message with explanation of the process #908
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13. (UI) replace RadioField by Radio component] #900
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