Re-add follow recommendations API (#7918)

* Re-add follow recommendations API

    GET /api/v1/suggestions

Removed in 8efa081f210d72ed450c39ac4cde0fd84fb3d3fb due to Neo4J
dependency. The algorithm uses triadic closures, takes into account
suspensions, blocks, mutes, domain blocks, excludes locked and moved
accounts, and prefers more recently updated accounts.

* Track interactions with people you don't follow

Replying to, favouriting and reblogging someone you're not following
will make them show up in follow recommendations. The interactions
have different weights:

- Replying is 1
- Favouriting is 10 (decidedly positive interaction, but private)
- Reblogging is 20

Following them, muting or blocking will remove them from the list,
obviously.

* Remove triadic closures, ensure potential friendships are trimmed

3 files changed, 22 insertions, 0 deletions

diff --git a/app/services/favourite_service.rb b/app/services/favourite_service.rb
index bc2d1547a..6e1ac3ba9 100644
--- a/app/services/favourite_service.rb
+++ b/app/services/favourite_service.rb
@@ -15,7 +15,10 @@ class FavouriteService < BaseService
     return favourite unless favourite.nil?
 
     favourite = Favourite.create!(account: account, status: status)
+
     create_notification(favourite)
+    bump_potential_friendship(account, status)
+
     favourite
   end
 
@@ -33,6 +36,11 @@ class FavouriteService < BaseService
     end
   end
 
+  def bump_potential_friendship(account, status)
+    return if account.following?(status.account_id)
+    PotentialFriendshipTracker.record(account.id, status.account_id, :favourite)
+  end
+
   def build_json(favourite)
     Oj.dump(ActivityPub::LinkedDataSignature.new(ActiveModelSerializers::SerializableResource.new(
       favourite,
diff --git a/app/services/post_status_service.rb b/app/services/post_status_service.rb
index 735985725..bad82051a 100644
--- a/app/services/post_status_service.rb
+++ b/app/services/post_status_service.rb
@@ -47,6 +47,8 @@ class PostStatusService < BaseService
       redis.setex("idempotency:status:#{account.id}:#{options[:idempotency]}", 3_600, status.id)
     end
 
+    bump_potential_friendship(account, status)
+
     status
   end
 
@@ -79,4 +81,9 @@ class PostStatusService < BaseService
   def redis
     Redis.current
   end
+
+  def bump_potential_friendship(account, status)
+    return if !status.reply? || account.following?(status.account_id)
+    PotentialFriendshipTracker.record(account.id, status.in_reply_to_account_id, :reply)
+  end
 end
diff --git a/app/services/reblog_service.rb b/app/services/reblog_service.rb
index 3c4e5847f..0ee8bac2f 100644
--- a/app/services/reblog_service.rb
+++ b/app/services/reblog_service.rb
@@ -24,6 +24,8 @@ class ReblogService < BaseService
     ActivityPub::DistributionWorker.perform_async(reblog.id)
 
     create_notification(reblog)
+    bump_potential_friendship(account, reblog)
+
     reblog
   end
 
@@ -41,6 +43,11 @@ class ReblogService < BaseService
     end
   end
 
+  def bump_potential_friendship(account, reblog)
+    return if account.following?(reblog.reblog.account_id)
+    PotentialFriendshipTracker.record(account.id, reblog.reblog.account_id, :reblog)
+  end
+
   def build_json(reblog)
     Oj.dump(ActivityPub::LinkedDataSignature.new(ActiveModelSerializers::SerializableResource.new(
       reblog,