MDL-12066 - Links in the HTML block are not recoded on backup and restore.

MDL-12066 - Links in the HTML block are not recoded on backup and restore.

This is messy because the links are hidden in the configdata column, which is serialized and base64encoded. So we have to untangle that, then ask the block whether there are any bits of $config that need to be fixed, then re-encode it before backup up. And reverse that on restore. It needs to remain base64 encoded in the backup file, so that the file format is backwards compatible.

I disucssed this with Eloy before doing it.

Merged from MOODLE_19_STABLE.