Save a query on issue and MR lists

`any?` on an AR relation performs a `SELECT COUNT`, which we don't need.

1. We are very likely to have issues or MRs, so the `SELECT COUNT` is
   often unnecessary.
2. Even where there are no items returned, the overhead of the
   `SELECT *` instead of `SELECT COUNT` is relatively small.

Calling `to_a` on the relation lets us use `Enumerable#any?`, which will
return immediately if there are objects returned.

app/views/shared/_issues.html.haml

-- if @issues.reorder(nil).any?
+- if @issues.to_a.any?
   - @issues.group_by(&:project).each do |group|
     .panel.panel-default.panel-small
       - project = group[0]

app/views/shared/_merge_requests.html.haml

-- if @merge_requests.reorder(nil).any?
+- if @merge_requests.to_a.any?
   - @merge_requests.group_by(&:target_project).each do |group|
     .panel.panel-default.panel-small
       - project = group[0]