Clean up styling of links in sidebar

For the right sidebar we override the link color: https://gitlab.com/gitlab-org/gitlab-ce/blob/bd30d0b5e70d1cea9bb839d9bf673b597c2e22d3/app/assets/stylesheets/pages/issuable.scss#L140-143

This leads to wrongly changing the style of dropdowns: https://gitlab.com/gitlab-org/gitlab-ce/blob/bd30d0b5e70d1cea9bb839d9bf673b597c2e22d3/app/assets/stylesheets/framework/dropdowns.scss#L266-270

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17519 we found a short-term fix. A long-term solution could be

• introduce .text-default and apply it to all links that should have default text color
• do not use <a> as a selector for dropdowns (prefer .menu-item for example)