[interface.html] storage cache

@gfwilliams @thyttan I'm thinking of making the file cache used in recorder (#3607) apply to all `Utils.readStorage()`, so that the health interface would benefit from it - users wouldn't have to wait to re-read the same data if they want to switch between a chart and a table.

I have half a PR but wanted to know, is this a change that'd be accepted?