Expand spec coverage and refactor the Account.find_ methods (#3485)

* Move specs for account finder methods to concern spec

* Move account finder methods to concern

* Improve spec wording

* Use more explicit comparison to ensure correct return value

* Add coverage for .find_local! and .find_remote!

* Add some methods to the finder

* Use arel on matching_username method

* Avoid ternary in matching domain method

* Simplify finder methods

* Use an AccountFinder class to simplify lookup

app/models/account.rb

@@ -42,6 +42,7 @@ class Account < ApplicationRecord
   MENTION_RE = /(?:^|[^\/[:word:]])@([a-z0-9_]+(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
 
   include AccountAvatar
+  include AccountFinderConcern
   include AccountHeader
   include AccountInteractions
   include Attachmentable
@@ -162,27 +163,6 @@ class Account < ApplicationRecord
   end
 
   class << self
-    def find_local!(username)
-      find_remote!(username, nil)
-    end
-
-    def find_remote!(username, domain)
-      raise ActiveRecord::RecordNotFound if username.blank?
-      where('lower(accounts.username) = ?', username.downcase).where(domain.nil? ? { domain: nil } : 'lower(accounts.domain) = ?', domain&.downcase).take!
-    end
-
-    def find_local(username)
-      find_local!(username)
-    rescue ActiveRecord::RecordNotFound
-      nil
-    end
-
-    def find_remote(username, domain)
-      find_remote!(username, domain)
-    rescue ActiveRecord::RecordNotFound
-      nil
-    end
-
     def triadic_closures(account, limit: 5, offset: 0)
       sql = <<-SQL.squish
         WITH first_degree AS (