Incentivize "link to page I need help with" input

[Cybr]

I've noticed users refraining from sharing the URLs of the pages they need help with; and if they do, they even sometimes omit the domain name.

It's very time-consuming guessing what kind of support the users need, as they often think we know exactly what "it doesn't work" means.

Sometimes, we need to ask for the link afterward when they require support, which defeats the purpose of having a private URL field.
And when I propose users sending the URLs via a private form, they're always sending it.

Thanks to the whole GDPR happening, users are more privacy-aware than ever. WordPress' track record of (theme/plugin/core) exploits doesn't help, either.

So, let's help them feel more confident in sharing the link, by:

1. Limiting the inputted URL visibility to plugin contributors, plugin support team members, and forum moderators only.
2. Informing the user of this by adding a little description beneath the input.

[Clorith]

So the field is only shown to users that are logged in so far, there's no way we will limit who can see the content of this field once they log in, that removes the concept of community support.

We can add a line stating that the field is only shown to logged in users though, that's not a bad idea and may help direct them towards using it.

[coffee2code]

In 8168:

Support Forums: Explicitly indicate that the URL provided when creaeting a support topic is only shown to logged-in users.

Props cybr, coffee2code.
Fixes #3685.

[coffee2code]

In 8169:

Support Theme: Remove border-bottom from site_url field and shift it to after new adjacent descriptive text.

See #3685.