Improve behaviour under multisite

[leonstafford]

noticed in #730 (comment)

@aramcap I could use your help in explaining the issue and desired behaviour to me, as I haven't looked at multisite in many years. I was under impression that WP2Static would be available for all subsites, via current activation...

[aramcap]

Hi @leonstafford, yes, I can help u. The plugin runs correctly in multisite mode (but not the addon). I've activated in the main menu, then I can use in each subsite separately. Also for addons is necessary this action: #730 (comment)

For plugin, the only error is with the path "/wp-content/plugins/". The plugin not returns error, but this path is not converted to static (resources returns httpcode 404). With the addon the error is the basepath is duplicated.