Add SDG phases cards management

These cards will be displayed in the SDG homepage.

Note there seems to be a strange behavior in cancancan. If we define
these rules:

can :manage, Widget::Card, page_type: "SDG::Phase"
can :manage, Widget::Card

The expected behavior is the first rule will always be ignored because
the second one overwrites it. However, when creating a new card with
`load_and_authorize_resource` will automatically add `page_type:
"SDG::Phase"`.

Similarly, if we do something like:

can :manage, Widget::Card, id: 3
can :manage, Widget::Card

Then the new card will have `3` as an ID.

Maybe upgrading cancancan solves the issue; we haven't tried it. For now
we're defining a different rule when creating widget cards.

app/assets/stylesheets/sdg_management/homepage/show.scss

@@ -0,0 +1,11 @@
+.phase-cards {
+  > header {
+    align-items: flex-start;
+    display: flex;
+
+    a {
+      @include regular-button;
+      margin-left: auto;
+    }
+  }
+}