TableBuilder: Extract custom action link methods to a new class

Move `#action_links`, `#action_link_method`, and
`#actions_after_policy_check` to a new class. They all depend on an
action and/or a model object, so it made sense to group these together.
Also, these should really be tested, and moved out of a private method
context. They now live in `TableBuilderHelper::CustomLinks`.

The advantage is that we now have these methods grouped together in a
separate module that can be tested separately.

Needed to change some things around now that they're in a class:

* `obj` and `action` are now instance variables
* In order to call Pundit's `policy` method, we have to call it directly
  on `Pundit`, since we're no longer in the context of a
  helper/controller/view.
* Create a `UserContext` that can be passed to Pundit based on the one
  created in `ApplicationController`.
* Rename some methods to make more sense in this new context.
* Move `actions_to_http_methods` to a class constant, so we're not
  redefining it in every call to `method_for_action`.
* Use `I18n.t` instead of `t` alone, otherwise getting the translation
  doesn't work.
* Move `TableBuilderHelper#polymorphic_url_parts` to a new class
  `TableBuilderHelper::URL`. This enables us to use it in both
  `CustomLinks` and the `TableBuilderHelper`. We can't use it from
  `CustomLinks` if it's a public method in `TableBuilderHelper`, and we
  can't use it in `TableBuilderHelper#tbody` if it's a class method in
  `TableBuilderHelper`.

There's a problem with the action symbols that aren't directly handled
in `actions_after_policy_check`. The `:show` action was passed by the
workbenches/show.html.slim template, but it doesn't appear in the
resulting output even though it should.

Refs #3479

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