Fix type of StatusBuilder::media_ids

The identifier in the JSON response to a POST to `/media` is a string,
as can be seen in the Attachment entity.

Even though that identifier has always been a string of numbers (at
least in Mastodon's case), it should not be assumed that it will
continue to be so in the future, so keeping it as a String seems more
prudent.

Additionally, this simplifies the process of posting a new status with
an attachment, as it is no longer necessary to parse the resulting
attachment's id to an u64.

src/status_builder.rs

@@ -26,7 +26,7 @@ pub struct StatusBuilder {
     pub in_reply_to_id: Option<u64>,
     /// Ids of media attachments being attached to the status.
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub media_ids: Option<Vec<u64>>,
+    pub media_ids: Option<Vec<String>>,
     /// Whether current status is sensitive.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub sensitive: Option<bool>,