Add step to check plugins page once a config restore has been performed

[ChrisC]

I had to rebuild my firewall as an upgrade broke it and it wouldn't boot (no idea how).
The restore process was straightforward, although getting the file from a USB stick failed.

After I restored the xml file using the web gui after installation, there was nothing obvious to say that there was anything wrong.
However, looking at the plugins page, there were loads of plugins highlighted in red (which must have meant that they were not part of the standard build and were missing).

It would be good to mention in the docs (plus a warning in the GUI?)  that this should be checked once a restore has been permformed, as well as rebooting opnsense once all the modules have been installed, as my config had not been applied.

Thanks!