Add ActivityPub actor representing the entire server (#11321)

* Add support for an instance actor

* Skip username validation for local Application accounts

* Add migration script to create instance actor

* Make Codeclimate happy

* Switch to id -99 for instance actor

* Remove unused `icon` and `image` attributes from instance actor

* Use if/elsif/else instead of return + ternary operator

* Add instance actor to fresh installs

* Use instance actor as instance representative

Use instance actor for forwarding reports, relay operations, and spam
auto-reporting.

* Seed database in test environment

* Fix single-user mode

* Fix tests

* Fix specs to accomodate for an extra `Account`

* Auto-reject follows on instance actor

Following an instance actor might make sense, but we are not handling that
right now, so auto-reject.

* Fix webfinger lookup and serialization for instance actor

* Rename instance actor

* Make it clear in the HTML view that the instance actor should not be blocked

* Raise cache time for instance actor as there's no dynamic content

* Re-use /about/more with a flash message for instance actor profile

spec/models/account_spec.rb

@@ -450,7 +450,7 @@ RSpec.describe Account, type: :model do
   describe '.domains' do
     it 'returns domains' do
       Fabricate(:account, domain: 'domain')
-      expect(Account.domains).to match_array(['domain'])
+      expect(Account.remote.domains).to match_array(['domain'])
     end
   end
 
@@ -665,7 +665,7 @@ RSpec.describe Account, type: :model do
           { username: 'b', domain: 'b' },
         ].map(&method(:Fabricate).curry(2).call(:account))
 
-        expect(Account.alphabetic).to eq matches
+        expect(Account.where('id > 0').alphabetic).to eq matches
       end
     end
 
@@ -732,7 +732,7 @@ RSpec.describe Account, type: :model do
         2.times { Fabricate(:account, domain: 'example.com') }
         Fabricate(:account, domain: 'example2.com')
 
-        results = Account.by_domain_accounts
+        results = Account.where('id > 0').by_domain_accounts
         expect(results.length).to eq 2
         expect(results.first.domain).to eq 'example.com'
         expect(results.first.accounts_count).to eq 2
@@ -745,7 +745,7 @@ RSpec.describe Account, type: :model do
       it 'returns an array of accounts who do not have a domain' do
         account_1 = Fabricate(:account, domain: nil)
         account_2 = Fabricate(:account, domain: 'example.com')
-        expect(Account.local).to match_array([account_1])
+        expect(Account.where('id > 0').local).to match_array([account_1])
       end
     end
 
@@ -756,14 +756,14 @@ RSpec.describe Account, type: :model do
           matches[index] = Fabricate(:account, domain: matches[index])
         end
 
-        expect(Account.partitioned).to match_array(matches)
+        expect(Account.where('id > 0').partitioned).to match_array(matches)
       end
     end
 
     describe 'recent' do
       it 'returns a relation of accounts sorted by recent creation' do
         matches = 2.times.map { Fabricate(:account) }
-        expect(Account.recent).to match_array(matches)
+        expect(Account.where('id > 0').recent).to match_array(matches)
       end
     end