Fix protocol-relative URL to use explicit HTTPS

[Copilot]

Protocol-relative URL //assets.okfn.org/images/icons/ajaxload-circle.gif causes mixed content warnings when served over HTTPS.

Changes

• list.js: Changed loading spinner image URL from //assets.okfn.org/... to https://assets.okfn.org/...
• README.md: Updated documentation in "Enabling HTTPS" section to reference HTTPS URL

// Before
listing_element.innerHTML = '<img src="//assets.okfn.org/images/icons/ajaxload-circle.gif" />';

// After  
listing_element.innerHTML = '<img src="https://assets.okfn.org/images/icons/ajaxload-circle.gif" />';

Original prompt

---

This section details on the original issue you should resolve

<issue_title>it should be https in place of http to avoid security issues</issue_title>
<issue_description>

s3-bucket-listing/list.js

Line 6 in 3d0774e

var BUCKET_URL = 'http://' + location.hostname + '.' + S3_REGION + '.amazonaws.com'; // e.g. just 's3' for us-east-1 region

</issue_description>

<agent_instructions>please check if this issue still exists and if so fix it.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@rufuspollock @maheshyamsani what security issues here exactly and do you want to do a PR? @rufuspollock OK, so can submit a PR.

• Fixes it should be https in place of http to avoid security issues #82

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