Pull request: home: improve mobileconfig http api

Merge in DNS/adguard-home from 2358-mobileconfig to master

Updates #2358.

Squashed commit of the following:

commit ab3c7a75ae21f6978904f2dc237cb84cbedff7ab
Merge: fa002e400 b4a35fa88
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Nov 25 16:11:06 2020 +0300

    Merge branch 'master' into 2358-mobileconfig

commit fa002e40004656db08d32c926892c6c820fb1338
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Nov 25 15:19:00 2020 +0300

    home: improve mobileconfig http api

internal/dhcpd/dhcphttp.go

@@ -509,6 +509,9 @@ func (s *Server) registerHandlers() {
 }
 
 // jsonError is a generic JSON error response.
+//
+// TODO(a.garipov): Merge together with the implementations in .../home and
+// other packages after refactoring the web handler registering.
 type jsonError struct {
 	// Message is the error message, an opaque string.
 	Message string `json:"message"`