Add loading indicator to machinery tab

[harmitgoswami]

The Machinery tab in the Translate frontend sometimes takes a while to load. There should be some sort of loading indicator until the tab gets populated with data.

This tab is the one in question:

[mathjazz]

Note that we have had loaders in Helper tabs in the past and dropping them improved perceived performance of the translate view. I'd only show loading indicators after a period of time, i.e. if the requests don't complete in N seconds.