Set provider prefix on creation and when editing a provider with no topics

[FionaLMcLaren]

What Issue Does This PR Cover, If Any?

Resolves #607

What Changed? And Why Did It Change?

Each provider has a file_name_prefix. This value is added to the file name
when the provider uploads a file. This file_name_prefix is necessary
to coordinate between our file storage and Azure's. However, if a provider
was to change their file_name_prefix when they had uploaded files,
it would break management of those existing uploaded files.

Before, we got around this by simply removing the field from the provider's
form. This meant that users could not set the value of file_name_prefix
when making a new provider or when editing a provider who had no uploaded
files.

This PR adds the ability to set the file_name_prefix value on the
provider form. The field will show on the form when the provider
is...

• A new instance
• One who has no topics, and no uploaded files

How Has This Been Tested?

RSpec and by verifying these test cases by hand...

• When I am creating a new provider, I can see and set
  the file_name_prefix field
• When I am editing a provider without topics, I can see
  and set the file_name_prefix field
• When I am editing a provider with topics, I cannot see
  and set the file_name_prefix field

Please Provide Screenshots

The provider form, with the file_name_prefix, for a new instance

The provider form, with the file_name_prefix, for editing an instance with no topics

Additional Comments

Some questions...

• In the form, the file_name_prefix field is not required. Should it
  be required?
• When the file_name_prefix was unavailable to set on the form,
  what was its value set to for a new Provider? Was it empty? Was
  it some default value?
• If file_name_prefix values were empty because of our
  past workaround, was this behaviour we wanted, or a tradeoff
  to avoid the issue?
• I have a provider with uploaded files, but their
  file_name_prefix is empty. Should we give the option to set
  the file_name_prefix here?
• Should we list providers’ topic in their show view to make it clearer
  why the file_name_prefix does not appear on their edit form and cannot
  be changed? Would we like to make it more explicit, too? - could we have a
  disabled field so we can still show the field on the form, but with a note
  why it isn’t editable?

The current provider Show view - their topics are not listed

[FionaLMcLaren]

Changed the placeholder here to be consistent with the rest of them. I think it is sufficient to have the placeholder give examples, as our help text says what the input should be