Add API modifiers to limit returned toots from public/hashtag timelines

to only those from local users; Add link to "extended information" to
getting started in the UI; Add defaults for posting privacy; Change
how publish button looks depending on posting privacy chosen

config/locales/simple_form.en.yml

@@ -23,6 +23,7 @@ en:
         note: Bio
         otp_attempt: Two-factor code
         password: Password
+        setting_default_privacy: Post privacy
         username: Username
       interactions:
         must_be_follower: Block notifications from non-followers