Add "block drafts" feature

Design doc:
https://docs.google.com/document/d/16AX1tKa1CGSWwZtbW42h1uHy8SEPuv1ZjT_oHxc0UUI/edit

Fixed: twpowertools:84
Change-Id: Ibb172113774c5e2cab14e3d87a178bafed85df0b
diff --git a/src/common/optionsPrototype.json5 b/src/common/optionsPrototype.json5
index 0132436..38cfaca 100644
--- a/src/common/optionsPrototype.json5
+++ b/src/common/optionsPrototype.json5
@@ -110,6 +110,13 @@
     context: 'options',
     killSwitchType: 'option',
   },
+  // #!if ['chromium', 'chromium_mv3'].includes(browser_target)
+  'blockdrafts': {
+    defaultValue: false,
+    context: 'options',
+    killSwitchType: 'option',
+  },
+  // #!endif
 
   // Experiments: