Include OpenVerse as a source for images

[bordoni]

OpenVerse seems to be a really good source for images.

API: https://api.openverse.engineering/v1/

[jonathanbossenger]

I'd like to help make this possible. Will work on a PR for this soon.

[jonathanbossenger]

I notice that the Openverse API requires OAuth2 registration to make more than 5 requests per hour:

Please note that there is a rate limit of 100 requests per day and 5 requests per hour rate limit in place for anonymous users. 

Authorized clients have a higher rate limit of 10000 requests per day and 100 requests per minute.

This is done by registering and getting an API key and secret and then using those to generate a valid token, to create API requests. Would we expect users to set this up manually, or should we make a provision for this in the plugin settings?