Expose image subscriptions similar to hides (#527).

[luckydonald]

As the count of hides of an image are provided,
similar shall be the (comment-)subscription count.

This is based on the changes for hides in ba90478 (know as eef9ec6 after rebase).

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My

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Changes made:

1. Migration: adds subscriptions_count integer not null default 0 to images and backfills it from image_subscriptions.
2. image.ex: added field :subscriptions_count, :integer, default: 0.
3. subscriptions.ex: added defoverridable create_subscription: 2, delete_subscription: 2 at the end of the quote block so callers can override with super/2.
4. philomena/images.ex: added create_subscription/2 and delete_subscription/2 overrides that call super then increment/decrement subscriptions_count atomically via Repo.update_all(inc: ...).
5. search_index.ex: added subscriptions: %{type: "integer"} to the mapping and subscriptions: image.subscriptions_count to as_json/1.
6. query.ex: added subscriptions to int_fields in anonymous_fields/0.
7. image_sorter.ex: added subscriptions to @allowed_fields.
8. _form.html.slime: added "Sort by subscriptions": :subscriptions after the hides entry.
9. json/image_view.ex: added subscriptions: image.subscriptions_count to the image JSON response.

Before you begin

• I understand my contributions may be rejected for any reason
• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software
• I understand my contributions are licensed under the GNU AGPLv3

• I understand all of the above

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[liamwhite]

What benefit does exposing the subscription count have?

[luckydonald]

@liamwhite Mostly symmetry to the hides, while I worked on #685.

[Meow]

I'll discuss this change with the booru mod team. They're ultimately the ones who decide what goes policy-wise, and exposing an extra number seems like a policy decision.