Move theme contextual CSS to the Seven theme

Follow-up to #2078803: Change contextual links background color to match the styleguide. →

Problem/Motivation

See discussion in: #2195695: Admin UIs on the front-end are difficult to theme →

Contextual links are an administrative UI component that appears on the frontend of sites. It's important that they are consistent with the Seven style guide and that other admin themes can control the look and feel.

Proposed resolution

Our CSS standards → define module CSS as: “the minimal styles needed to get the module's functionality working.”
Theme CSS is defined as: “extra styles to make the module's functionality aesthetically pleasing. ”
Move the theme styling into the Seven theme
Add a library alter hook to load the admin theme CSS. Example: 🐛 Node preview bar has usability issues, is difficult to use on mobile, not usable without Bartik, and does not align with the Seven style guide and current toolbar designs Postponed: needs info

Remaining tasks

Write the patch.
Test.

User interface changes

None

API changes

None

Data model changes

None