Give purpose to previously unused on_budget_investments scope

The "on_budget_investments" scope in Activity has never been used
anywhere in the codebase. It was introduced in commit d9d38482b3
("extends Activity to include Investment valuations") but no references
were ever added.

Instead of removing it, we make use of the scope by adding the missing
"Budget investments" filter to the admin Activity section. This aligns
it with the rest of the activity filters and gives the scope the purpose
it was originally intended for.

config/locales/en/admin.yml

@@ -62,6 +62,7 @@ en:
         filter: Show
         filters:
           all: All
+          on_budget_investments: Investment projects
           on_comments: Comments
           on_debates: Debates
           on_proposals: Proposals

config/locales/es/admin.yml

@@ -62,6 +62,7 @@ es:
         filter: Mostrar
         filters:
           all: Todos
+          on_budget_investments: Proyectos de gasto
           on_comments: Comentarios
           on_debates: Debates
           on_proposals: Propuestas