Adding alt_text and custom_thumbnail to post form. Fixes #1513

[dessalines]

This adds the recently added alt_text and custom_thumbnail fields to creating and editing posts:

• The alt_text field is only shown after you've added a url that's an image, matching lemmy-ui's behavior.
• The postView.post.alt_text is now used as the contentDescription fields for pictrs images.
• There might be some issues with custom_thumbnail, but that's a back-end problem still.

Example:

https://github.com/LemmyNet/jerboa/assets/930215/06b065cc-207c-48ba-baa8-dbe599f7792b