Fix for #1800: Don't clear "server is offline" message in update pages.

Issue was caused by post `is-server-online` AJAX requests to the distributed server succeeding.  Solution was to control when the `is-server-online` check is done within each update page, and to NOT do any update checking if server is offline (save resources, don't reset error state).

This changes includes:
* Moving `is-server-online` code into each `updates` page.
* Only calling `updatesStart` (to start the update log checking) if the server is online.
* Cleaning up the `update_software.js` file (removing unnecessary callbacks, wrapping some strings in `gettext` / `sprintf`)