Allow managers to read investment suggestions

When creating a budget investment with an unverified manager (for
example, a manager who isn't part of the local census), there's a
request to `Budgets::InvestmentsController#suggest`. Since the manager
isn't verified, suggestions can't be obtained.

There are serveral ways to fix this problem:

* Add a `suggest` action to Management::Budgets::InvestmentsController,
doing the same thing the main `suggest` action does.
* Give unverified users permission to access investment suggestions
* Give managers permission to access investment suggestions

I've chosen the last one because I thought it was simple and only
changed existing behaviour for managers, but any other solution would be
as valid. I haven't added the `phase: "accepting"` condition to keep it
simple, since a read-only action like this one in the management portal
isn't gonna create security risks.

app/models/abilities/manager.rb

@@ -0,0 +1,11 @@
+module Abilities
+  class Manager
+    include CanCan::Ability
+
+    def initialize(user)
+      merge Abilities::Common.new(user)
+
+      can :suggest, Budget::Investment
+    end
+  end
+end

app/models/ability.rb

@@ -14,6 +14,8 @@ class Ability
         merge Abilities::Administrator.new(user)
       elsif user.moderator?
         merge Abilities::Moderator.new(user)
+      elsif user.manager?
+        merge Abilities::Manager.new(user)
       else
         merge Abilities::Common.new(user)
       end