Pull request: 2826 auth block

Merge in DNS/adguard-home from 2826-auth-block to master

Updates #2826.

Squashed commit of the following:

commit ae87360379270012869ad2bc4e528e07eb9af91e
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 27 15:35:49 2021 +0300

    home: fix mistake

commit dfa2ab05e9a8e70ac1bec36c4eb8ef3b02283b92
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 27 15:31:53 2021 +0300

    home: imp code

commit ff4220d3c3d92ae604e92a0c5c274d9527350d99
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 27 15:14:20 2021 +0300

    home: imp authratelimiter

commit c73a407d8652d64957e35046dbae7168aa45202f
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 27 14:20:17 2021 +0300

    home: fix authratelimiter

commit 724db4380928b055f9995006cf421cbe9c5363e7
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Apr 23 12:15:48 2021 +0300

    home: introduce auth blocker

openapi/CHANGELOG.md

@@ -4,7 +4,12 @@
 
 ## v0.106: API changes
 
-## New `"private_upstream"` field in `POST /test_upstream_dns`
+### New response code in `POST /control/login`
+
+* `429` is returned when user is out of login attempts.  It adds the
+  `Retry-After` header with the number of seconds of block left in it.
+
+### New `"private_upstream"` field in `POST /test_upstream_dns`
 
 * The new optional field `"private_upstream"` of `UpstreamConfig` contains the
   upstream servers for resolving locally-served ip addresses to be checked.

openapi/openapi.yaml

@@ -1078,6 +1078,12 @@
       'responses':
         '200':
           'description': 'OK.'
+        '400':
+          'description': >
+            Invalid username or password.
+        '429':
+          'description': >
+            Out of login attempts.
   '/logout':
     'get':
       'tags':