feature(menus/site): add "Send feedback" link for reporting bugs/ideas
feature(appcache): poll for appcache updates

This is configurable so you can poll more frequently during
development and hopefully avoid the "double refresh" problem
that appcache is historically associated with.

Example for polling every 1000ms:
```json
{
  "appcache": {
    "maxAge": 1000
  }
}

Polling is inefficient at scale but when you're talking about
a single person during development, it's not a problem.
fix(headings): elgg-heading-main did not need left-float any more
fix(blog): blog.edit state resolves now