Migrate post_store to sqlx (#19241)

* Migrate post_store to sqlx

There are several queries which should improve further
if re-written with squirrel. HW tickets will be opened
for those.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/ctnryw9mga7fu9kc16xsap4phuw

```release-note
NONE
```

* Remove struct

```release-note
NONE
```

* fix tests

```release-note
NONE
```

model/utils.go

@@ -135,6 +135,11 @@ func (si *StringInterface) Scan(value interface{}) error {
 	return errors.New("received value is neither a byte slice nor string")
 }
 
+// Value converts StringInterface to database value
+func (si StringInterface) Value() (driver.Value, error) {
+	return json.Marshal(si)
+}
+
 var translateFunc i18n.TranslateFunc
 var translateFuncOnce sync.Once
 

store/storetest/post_store.go

@@ -1818,7 +1818,7 @@ func testPostStoreGetPostBeforeAfter(t *testing.T, ss store.Store) {
 	require.NoError(t, err)
 
 	rPost3, err := ss.Post().GetPostAfterTime(channelId, o2a.CreateAt, false)
-	require.Empty(t, rPost3, "should return no post")
+	require.Empty(t, rPost3.Id, "should return no post")
 	require.NoError(t, err)
 }