Show more hints/warnings in the admin

[helen]

There are a few technical requirements for the plugin which should be handled in case a user still decides to try activating the plugin:

• PHP 5.3+, mostly for __DIR__ and namespace reasons (for now). This is handled by WordPress core's sandboxing right now, as it fatals and is subsequently deactivated. It's possible nothing else needs to be done here other than trying to advance the relevant core ticket.
• WordPress 4.9+, for the usage of CodeMirror as bundled. The editor likely still (mostly) works anyway, though there may be some JS errors, but hasn't been tested. It almost certainly requires 4.6+ for language pack reasons. Not sure what the plugin should do - I believe it would have to be installed manually rather than from the plugin repo as it is, so do we need to worry much about a warning?
• Rewrites enabled, so that WordPress can actually catch the request to /ads.txt. Perhaps a warning on the plugins page/row and on the edit screen?
• Site URL must not contain a path, as per spec this is a root-level file and the plugin only attempts to serve it there. Perhaps also a warning with explanation on the plugins page/row and on the edit screen?

Since this plugin has a bit of a smaller audience this wasn't 1.0 priority, but we should still aim for the best possible experience.