Allow outer partial access when capturing

[kaspth]

Due to rendering happening in the same ActionView::Base instance,
the @partial instance variable was effectively shared between all
render calls.

As such, doing this:

render "card" do |cp|
  cp.content_for :title, partial.content_for(:title)
end

…wouldn't work because partial would point to the inner cp.

This commit fixes it so we no longer shadow the partial method.

Fixes #38

[kaspth]

Got back to check more on this, but looks like it's causing some empty content areas. I'll fix.

[kaspth]

Ok, I replaced the previous "fix" with a stack based approach that's more reliable.

With the previous version of the fix rendering creative_concepts#show would render themes/light/_page.html.erb, which wouldn't properly reset to the outer partial after a render call. Like this:

<%= render 'account/shared/title' do |cp| %>
  <% cp.content_for :title, partial.content_for(:title) %>
  <% cp.content_for :actions, partial.content_for(:actions) %>
<% end %>

<%= render 'account/shared/notices' %>

<div class="space-y-8 py-4 xl:py-8 xl:px-8">
  <%# ⚠️ After the `render 'account/shared/title' render, `partial` would really point to `cp` even though it shouldn't %>
  <%= partial.yield :body %>
</div>

The stack based approach fixes this.