Ability to control the output of AccountView.pleroma.relationship ([:account][:pleroma][:relationship]) in statuses and notifications in order to improve the rendering performance.

Corresponding FE issue: pleroma-fe#819 (closed) Meta-issue: pleroma-meta#33

Note: default behaviour is to keep rendering user relationships in statuses and notifications ([:extensions, output_relationships_in_statuses_by_default] setting being true) in order to keep compatibility with older and current versions of PleromaFE. With this setting, with_relationships param is ignored.

Once PleromaFE starts sending with_relationships=true for all status/notification-related requests (index/show), we could change [:extensions, output_relationships_in_statuses_by_default] to false (the only BE change that would be required). However, doing that would result in breaking the compatibility with older versions of PleromaFE (!) since they know nothing about with_relationships param.

NOTE for pleroma-fe#819 (closed) developers:

when implementing the compatibility with relationships-less statuses/notifications, please change config :pleroma, :extensions, output_relationships_in_statuses_by_default: true to config :pleroma, :extensions, output_relationships_in_statuses_by_default: false