Improve admin notice usability

[jrfnl]

Originally the admin notice would only be displayed on the plugins page and only once.
For multisite, this meant that unless an admin actually visited the plugins page, the notice might go unseen for weeks, if not months.
I've changed this in the multisite PR #30 in a way that the notice will now show on the first admin page visited by an author/editor/admin (so the lower 'ranks' can notify an admin if they see it).

I'd suggest we think about further improvements to the usability of this along the lines of:

• keep showing the notice until someone with admin access has visited the plugins page ?
• maybe offer an ignore option ? for non-admins, we'd need to keep track of who ignored it to not show it, if an admin ignores it, the message (transient) should be deleted.
• maybe add 'please notify your website administrator' to the text shown to non-admins ?