Add an account endpoint for ActivityPub and link to it on HTML profile pages

app/views/api/activitypub/accounts/show.rabl

@@ -0,0 +1,10 @@
+object @account
+
+node(:'@context')         { 'https://www.w3.org/ns/activitystreams' }
+node(:type)               { 'Person' }
+node(:id)                 { request.original_url }
+node(:url)                { |account| TagManager.instance.url_for(account) }
+node(:name)               { |account| account.display_name }
+node(:preferredUsername)  { |account| account.username }
+node(:summary)            { |account| account.note }
+node(:icon)               { |account| full_asset_url(account.avatar.url(:original)) }