#6409689 Add Reader Featured image alt text

Changes proposed in this Pull Request

The proposed code adds alt text to post Featured image in Reader at read/feeds/[xxxxxxxx]/posts/[xxxxxxxxx]:

The code tries to extract the alt text from post.attachments object first. If there is no post attachment available (e.g. when the Featured image was uploaded before the time when the post was created) – OR – if the existing post attachment doesn’t have any alt text, the post title will be used as the alt text instead.

Testing instructions

1. Create a new post with Featured image.
2. Add alt text to the Featured image through Media section in Calypso.
3. Navigate to Reader at https://wordpress.com/read.
4. Open the new post in Reader. You should be at read/feeds/[xxxxxxxx]/posts/[xxxxxxxxx] (not on the post page itself).
5. Open the Console and take a look if the Featured image has its alt text.
6. Try to remove alt text from the image through the Media section and go back to read/feeds/[xxxxxxxx]/posts/[xxxxxxxxx].
7. When you open the Console this time, the alt text of the Featured image should be the same as the site title.

Related to #49226