[Part 1] 5276 Remove 'build-page' from 'ide-terminal' element

What does this MR do?

CE Port => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29331

This MR is part of the larger WIP MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12239.

The styles applied to .build-page .top-bar were mostly unecessary for the IDE terminal and may have also been causing a strange line to show up in the terminal.

This removes the build-page selector in the Web IDE terminal and extracts some common styling into a mixin.

Screenshots

Pipeline tab (looks the same)	Terminal tab (looks the same)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Links

• Part of #5276 (closed)