Improved image and URL metadata handling

[roelofr]

• URLs no longer need to be alive, just valid
• Use Laravel's HTTP library, not file_get_contents and streams
• Replaced URL-image-upload mimetype test with test on HTTP response
• Added a bunch of tests for the controller

[tractorcow]

This issue caused a regression when attaching links that have image metadata.

Something went wrong: Found extra param `imageUrl`

the meta.image.url field was just replaced with meta.imageUrl, but this was not reflected in the config.