Add user setting for default post visibility

Changelog.md

app/assets/javascripts/app/pages/settings.js

app/controllers/users_controller.rb

app/helpers/aspect_global_helper.rb

app/helpers/interim_stream_hackiness_helper.rb

app/helpers/publisher_helper.rb

app/models/user.rb

app/views/aspects/_aspect_dropdown.html.haml

app/views/publisher/_aspect_dropdown.html.haml

app/views/publisher/_publisher.html.haml

app/views/users/_edit.haml

app/views/users/_post_default.html.haml

app/views/users/_post_default.mobile.haml

config/locales/diaspora/en.yml

db/migrate/20160829170244_add_post_default_to_aspects.rb

db/schema.rb

features/desktop/change_settings.feature

features/step_definitions/aspects_steps.rb

lib/stream/base.rb

lib/stream/multi.rb