Add ignore version flag to generate artifacts-index

[tashima42]

To maintain compatibility with older tests in rancher/rancher we need to maintain some artifacts for a non-prime version on the prime artifacts bucket, this is not a problem as long as we don't show it on our index.

• add an ignore version flag to generate artifacts-index

  release generate rancher artifacts-index -w ./tmp -i v2.6.4

• show files by default and hide them with javascript, to support users who block javascript
• minor style changes