A template variable called "value" can leak into the field rendering

If a variable `{{ value }}` is present outside the `{% formfield %}`, and the formfield has empty data (e.g. for an unbound form), then the value from the outerscope is used to render the field.

No files or tests are named. Start by reproducing the issue with an outer {{ value }} variable and an empty formfield in an unbound form, then trace the formfield rendering entry point. Done means the outer value no longer affects an empty field, with a regression test covering this template case.

Written by the indexing model from the issue text.