Updated _schedule_at to use local time when _interval is set

[abhi1693]

Closes: #11645

When recurs every is set but schedule at is left empty, local time displayed on the form will be used instead.

[jeremystretch]

Could you replicate this change to ReportForm as well please?

[abhi1693]

Is there any specific reason that ScriptForm has fields with underscore and ReportForm does not?

[jeremystretch]

Yes; reports don't take any user input, so we're free to declare whatever form fields we want. Scripts, however, can declare their own form fields with any names, which could potentially conflict with the built-ins we add. So, we prepend the built-in field names with underscores as it makes a collision much less likely and helps to differentiate them from the script's own fields.

[jeremystretch]

Thanks for taking this!