[UX] Merge the "List fields" and "Used in Views" pages.

[klonos]

/admin/reports/fields
/admin/reports/fields/views-fields

Notwithstanding #3318, which is a bug, I believe that the purpose of these pages are to show where fields are being used, so that site admins can for example make better-informed decisions (like can a field be altered/removed etc.). I don't see how having a separate page that lists where a field is used in content types vs another for views helps.

We should merged these tables into a single page.

I've personally never seen the purpose of, or used, the second page, but I agree they should be merged.