Reorder the fields of the Entry struct to save some memory

Given that there is always a ton of `Entry` floating around, reordering its
field to take less space is a quick/simple way to reduce miniflux' memory
consumption.

I kept the `ID` field as the first member, as I think it's the most important
one, and moving it somewhere else would drown it in other fields.

Anyway, this still provides a reduction of 32 bytes per Entry:

```console
$ fieldalignment  ./client/model.go 2>&1 | grep 203
~/v2/client/model.go:203:12: struct with 280 pointer bytes could be 240
$ fieldalignment  ./client/model.go 2>&1 | grep 203
~/v2/client/model.go:203:12: struct with 248 pointer bytes could be 240
$
```

The same optimisation pass could be applied to other structs, but since they
aren't present in obviously great numbers during miniflux' life cycle, it would
likely require some profiling to see if it's worth doing it.

client/model.go

@@ -202,24 +202,24 @@ type Feeds []*Feed
 // Entry represents a subscription item in the system.
 type Entry struct {
 	ID          int64      `json:"id"`
-	UserID      int64      `json:"user_id"`
-	FeedID      int64      `json:"feed_id"`
-	Status      string     `json:"status"`
+	Date        time.Time  `json:"published_at"`
+	ChangedAt   time.Time  `json:"changed_at"`
+	CreatedAt   time.Time  `json:"created_at"`
+	Feed        *Feed      `json:"feed,omitempty"`
 	Hash        string     `json:"hash"`
-	Title       string     `json:"title"`
 	URL         string     `json:"url"`
 	CommentsURL string     `json:"comments_url"`
-	Date        time.Time  `json:"published_at"`
-	CreatedAt   time.Time  `json:"created_at"`
-	ChangedAt   time.Time  `json:"changed_at"`
+	Title       string     `json:"title"`
+	Status      string     `json:"status"`
 	Content     string     `json:"content"`
 	Author      string     `json:"author"`
 	ShareCode   string     `json:"share_code"`
-	Starred     bool       `json:"starred"`
-	ReadingTime int        `json:"reading_time"`
 	Enclosures  Enclosures `json:"enclosures,omitempty"`
-	Feed        *Feed      `json:"feed,omitempty"`
 	Tags        []string   `json:"tags"`
+	ReadingTime int        `json:"reading_time"`
+	UserID      int64      `json:"user_id"`
+	FeedID      int64      `json:"feed_id"`
+	Starred     bool       `json:"starred"`
 }
 
 // EntryModificationRequest represents a request to modify an entry.