Store config data serialized (#10651)

* Store config data serialized
This way we can store null, booleans, and more reliably.

* Use model to get the mutated output.

* fix whitespace and unused function

* use json_encode/decode and casts
migration to transfer

* json_encode JSON_UNESCAPED_SLASHES

* Use JSON_UNESCAPED_SLASHES.  That is only relevant if you are printing into an HTML page.

* pre-encode the seed...

* filter other fields besides config_value

tests/ConfigTest.php

@@ -126,7 +126,7 @@ class ConfigTest extends LaravelTestCase
 
         $key = 'testing.persist';
 
-        $query = Eloquent::DB()->table('config')->where('config_name', $key);
+        $query = \App\Models\Config::query()->where('config_name', $key);
 
         $query->delete();
         $this->assertFalse($query->exists(), "$key should not be set, clean database");