Add privacy disclaimer

Add variable for text of privacy disclaimer

app/assets/javascripts/components/locales/en.jsx

@@ -41,6 +41,7 @@ const en = {
   "compose_form.sensitive": "Mark media as sensitive",
   "compose_form.spoiler": "Hide text behind warning",
   "compose_form.private": "Mark as private",
+  "compose_form.privacy_disclaimer": "Warning: Private posts are not encrypted, and could be read or boosted by instances or people who do not respect post privacy. This is not true privacy. Do not post senstive information."
   "compose_form.unlisted": "Do not display in public timeline",
   "navigation_bar.edit_profile": "Edit profile",
   "navigation_bar.preferences": "Preferences",