Covers from multiple sources RFC

Existing sources for covers

Accessing book cover images - a guide for libraries
- Note: use of this from outside New Zealand is not permitted.

This is to get the basics working, with an eye to improving things later.

Method:

The page requiring an image requests a URL from Koha, e.g. opac-cover.pl?biblionumber=x
That script looks in the cache^[1] to see if it's seen a request for this before.
1. If so: it proxies a request of the URL straight out, finishes.
2. If not: it walks through the list of enabled cover services^[2] checking each one to see if it can supply a cover
If a service can provide the cover, it will proxy the request through and finish.
If no cover can be found, a default 1x1 image will be supplied (see $noimage in C4::Images)

Notes:

The cache will store what service successfully provided an image, it won't store the images themselves. It will use MySQL and possibly memcache.
These services will be provided in a nice plugin type fashion to make it easy to add new ones.

Implement Koha::CoverImages. This will:
- Load a set of modules for the available cover image services
- Take a biblio number and ask each module if it can supply an image URL for that
- When it finds one that can, it returns the URL.
- If it can't, it will return undef to indicate this.
- It will also have a method to output a 1x1 image to indicate that no cover was found.
Implement Koha::Network. This will contain helper things related to networking. In this case:
- A method to aid in proxying a URL out as the result of a request. This will also allow browser caching headers to be set.
Implement opac-covers.pl
- It will take a biblionumber and an option as to whether the image requested is a thumbnail
- Ask Koha::CoverImages for the URL
- If a URL was provided, use the method in Koha::Network to send the contents of the URL to the browser
- If undef was returned, output the 'no cover' image from Koha::CoverImages
Implement catalogue/covers.pl which will be virtually identical to opac-covers.pl
Implement modules for each of the services above. Each module will:
- Be able to determine if it can supply an image by looking at the sysprefs, and then querying the service if necessary.
- Provide a thumbnail form of the image as well as the full-size one
- Provide a URL to the image.
Modify opac-detail, opac-results, opac-readingrecord, opac-user, opac-shelves, opac-showreviews, catalogue/detail, catalogue/results so that they use the new method instead of the old.

Ordering of the sources - code for this is apparently coming in Bug 8726
Returning HTML instead of an image, this can be pulled in with an iframe and will allow things like having the image clickable.
Caching of images from services that allow that to happen.
Providing a default "not found" image if nothing can be found.