Refactor select css. Make selects same width

app/assets/stylesheets/generic/selects.scss

app/assets/stylesheets/pages/groups.scss

app/assets/stylesheets/pages/issues.scss

app/assets/stylesheets/pages/merge_requests.scss

app/assets/stylesheets/pages/tree.scss

app/views/groups/group_members/_group_member.html.haml

app/views/projects/issues/index.html.haml

app/views/projects/project_members/_project_member.html.haml

app/views/projects/protected_branches/index.html.haml