Reuse code between feed components

They were all following the same format.

Note we need to group the `see_all` translation keys together (the same
way it's done with the `most_active` keys) so we don't have an unused
translation warning.

We're also moving the "see all" link in processes outside the feed
content; the same way it's done in debates and proposals and removing
unnecessary classes in the processes feed: the column class is causing
the processes not to be aligned with the debates above them, and the
margin bottom is not needed because the margin of the footer is already
enough.

spec/system/admin/homepage/homepage_spec.rb

@@ -106,7 +106,7 @@ describe "Homepage", :admin do
       visit root_path
 
       expect(page).to have_content "Open processes"
-      expect(page).to have_css(".legislation_process", count: 3)
+      expect(page).to have_css(".legislation-process", count: 3)
     end
 
     xscenario "Deactivate"