ui: Add Accept header to retrieve JSON errors

frontend/ui/src/components/ExerciceFlags.svelte

@@ -43,6 +43,7 @@
          "submit/" + exercice.id,
          {
              method: "POST",
+             headers: {'Accept': 'application/json'},
              body: JSON.stringify(responses),
          }
      )

frontend/ui/src/components/ExerciceHints.svelte

@@ -47,6 +47,7 @@
          "openhint/" + exercice.id,
          {
              method: "POST",
+             headers: {'Accept': 'application/json'},
              body: JSON.stringify({ id: hint.id }),
          }
      )

frontend/ui/src/components/FlagKey.svelte

@@ -41,6 +41,7 @@
          "wantchoices/" + exercice_id,
          {
              method: "POST",
+             headers: {'Accept': 'application/json'},
              body: JSON.stringify({ id: Number(flag.id) }),
          }
      )

frontend/ui/src/components/TeamChangeName.svelte

@@ -48,6 +48,7 @@
 
      const response = await fetch('chname', {
 	 method: "POST",
+         headers: {'Accept': 'application/json'},
 	 body: JSON.stringify({newName: newTeamName}),
      });