Conventional location and name for custom field (#2842)

There is an inconsistency between the `administrate:field` generator and the
custom field `HasManyVariant` present in the demo app. They use different
conventions for naming and file paths.

After thinking about which of the two styles is right or wrong, I think the
generator is doing the right thing. This PR moves `HasManyVariant` and
renames it `HasManyVariantField`, in order to follow the convention set out
by the generator (which is also the one shown in the docs).

Notes:
- The field class is generated under `app/fields`. The location means it's
  autoloaded and we don't need a `require`.
- The `Field` suffix may seem annoying, but it's not different from
  controllers, helpers, etc, and removes the need for a namespace
  (eg: `Field::HasManyVariant`).
- This establishes a difference between the official field types
  (eg: `Administrate::Field::String`) and custom ones (eg:
  `MyStringField`), but I think this is fine.

Tangentially I also noticed that, if for any reason a user puts the template
files in the wrong place, this is difficult to debug as Administrate will
just fail silently, rendering empty templates. This is due to the changes
introduced at #2467. This
should not be an issue if using the generator, but might be if creating the
field manually.

Author: pablobm

spec/example_app/app/dashboards/customer_dashboard.rb

@@ -1,4 +1,3 @@
-require "administrate/field/has_many_variant"
 require "administrate/base_dashboard"
 
 class CustomerDashboard < Administrate::BaseDashboard
@@ -9,7 +8,7 @@ class CustomerDashboard < Administrate::BaseDashboard
     lifetime_value: Field::Number.with_options(prefix: "$", decimals: 2, sortable: false),
     name: Field::String,
     orders: Field::HasMany.with_options(limit: 2, sort_by: :id),
-    log_entries: Field::HasManyVariant.with_options(limit: 2, sort_by: :id),
+    log_entries: HasManyVariantField.with_options(limit: 2, sort_by: :id),
     updated_at: Field::DateTime,
     kind: Field::Select,
     territory: Field::BelongsTo.with_options(

spec/example_app/app/fields/has_many_variant_field.rb

@@ -0,0 +1,3 @@
+class HasManyVariantField < Administrate::Field::HasMany
+  # Only here to test that this works out of the box
+end

spec/example_app/lib/administrate/field/has_many_variant.rb

@@ -1,6 +1,5 @@
 require "rails_helper"
 require "administrate/field/has_many"
-require "administrate/field/has_many_variant"
 
 RSpec.describe Administrate::ApplicationHelper do
   describe "#find_partial_prefix" do
@@ -13,7 +12,7 @@
 
     context "when the field does not have a partial and the superclass does" do
       it "returns the superclass prefix" do
-        field = Administrate::Field::HasManyVariant.new(:name, "hello", :show)
+        field = HasManyVariantField.new(:name, "hello", :show)
         expect(find_partial_prefix(field)).to eq("fields/has_many")
       end
     end