Link to comparison of environment with default branch on environment list

Problem

After the following issues have been implemented, we should make it easier to summon up the right information in the comparison view, being on the environments list.

• https://gitlab.com/gitlab-org/gitlab-ce/issues/23508
• https://gitlab.com/gitlab-org/gitlab-ce/issues/31421

Proposal

Add a button to summon up the comparison view VS the default branch from the environment list view

Design

• Compare by default against "default branch"
• Icon button: comparison => http://gitlab-org.gitlab.io/gitlab-svgs/
• Tooltip: Compare environment