Adding a block model and filter mentions from blocked users (fix #60)

app/models/block.rb

@@ -0,0 +1,7 @@
+class Block < ApplicationRecord
+  belongs_to :account
+  belongs_to :target_account, class_name: 'Account'
+
+  validates :account, :target_account, presence: true
+  validates :account_id, uniqueness: { scope: :target_account_id }
+end