Simplify code to toggle investment selection

This way it'll be easier to change the link/button used to toggle the
selection.

Note that the conditions in the view seem to be different because we no
longer include the `selected?` condition when rendering the link/button.
However, an investment can only be selected if it's feasible and its
valuation is finished, so writing something like this would have been
redundant:

```ruby
can?(:toggle_selection, investment) &&
  (selected? || investment.feasible? && investment.valuation_finished?)
```

The reason why the previous code was using the `selected?` condition was
to check whether to render the link/button to select or to deselect an
investment. We're now doing that in the Ruby part of the component.

app/components/admin/budget_investments/toggle_selection_component.html.erb

@@ -1,16 +1,5 @@
-<% if investment.selected? %>
-  <%= link_to_if can?(:toggle_selection, investment),
-                 t("admin.budget_investments.index.selected"),
-                 path,
-                 method: :patch,
-                 remote: true,
-                 class: "button small expanded" %>
-<% elsif investment.feasible? && investment.valuation_finished? %>
-  <% if can?(:toggle_selection, investment) %>
-    <%= link_to t("admin.budget_investments.index.select"),
-                path,
-                method: :patch,
-                remote: true,
-                class: "button small hollow expanded" %>
-  <% end %>
+<% if can?(:toggle_selection, investment) && investment.feasible? && investment.valuation_finished? %>
+  <%= link_to text, path, method: :patch, remote: true, class: html_class %>
+<% elsif selected? %>
+  <%= selected_text %>
 <% end %>

app/components/admin/budget_investments/toggle_selection_component.rb

@@ -1,6 +1,7 @@
 class Admin::BudgetInvestments::ToggleSelectionComponent < ApplicationComponent
   attr_reader :investment
   use_helpers :can?
+  delegate :selected?, to: :investment
 
   def initialize(investment)
     @investment = investment
@@ -8,6 +9,18 @@ class Admin::BudgetInvestments::ToggleSelectionComponent < ApplicationComponent
 
   private
 
+    def text
+      if selected?
+        selected_text
+      else
+        t("admin.budget_investments.index.select")
+      end
+    end
+
+    def selected_text
+      t("admin.budget_investments.index.selected")
+    end
+
     def path
       toggle_selection_admin_budget_budget_investment_path(
         investment.budget,
@@ -20,4 +33,12 @@ class Admin::BudgetInvestments::ToggleSelectionComponent < ApplicationComponent
         page: params[:page]
       )
     end
+
+    def html_class
+      if selected?
+        "button small expanded"
+      else
+        "button small hollow expanded"
+      end
+    end
 end