Adds `# frozen_string_literal: true` to spec/models ruby files

When looking for upcoming milestones, we compare the milestone's due
date to `NOW()`. However, the due date is a date and `NOW()` is a
timestamp, and in Postgres, timestamps can't extend as far in the future
as dates can.

This test exposes the issue.

Add project when creating milestone in specs

We validate milestone is from the same
project/parent group as issuable ->
we need to set project in specs correctly

Improve methods names and specs organization

Adds to search ILIKE search for milestones title in:
  - Milestones dashboard
  - Group milestones page
  - Project milestones page

Instead of querying relations into ids we just pass them to the model
scope because the scope supports it now.

Also changes other calls to `Milestone.for_projects_and_groups`

Refactored to use Rails 5 ActiveRecord `or` so that it can handle ids,
objects, array of objects, or relations properly.

Also update banzai tests to use reference_link_text