Prevented brief flash of all tab panels

This is way short of being ready for a merge, but starting a pull request for discussion

I added this change to prevent the brief display of all tab panels when page is loading. Having done it, and also reading through the sass which ensures equal height of tabs, I wonder if the majority of lines 65-81 should be provided as a mixin of o-tabs `oTabsPanel(selector)`
This should now be `o--if-js`