Project, Group information sections: Add style for text highlighted with `<mark>`

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What does this MR do and why?

!161091 (merged) added a style for the <mark> tag used in Markdown. But that style was not being applied to the description sections in projects and groups.

In order to reduce duplication, extract that style into into a new mixin, str-highlighted.

Include that mixin from {group, search}.scss, and modify {project, projects, groups}.scss so that text wrapped in <mark> tags in project descriptions is rendered consistently.

Addresses #476690 (closed).

Changelog: fixed

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Screenshots or screen recordings

Before	After
After these changes, long text continues to be truncated with Read more in the Project view.
And the Search and Global Search highlights keep their style.
mark tags inside Markdown comments have the same style

How to set up and validate locally

Setup

1. Load an existing project
2. Go to its Project settings (Settings > General on the left-hand menu).
3. Add some text containing a <mark> tag in the Project description (optional) field. For example, "Here is some h<mark>ighlighted tex</mark>t".
4. Hit Save changes

Check project view

1. Go to project view.
2. See Project information on the right.

Check projects list

1. Load Your work > Projects view (e.g. http://gdk.local:3000/dashboard/projects).
2. Check project description under the project's title.

frontend UX Paper Cuts

Addresses #476690 (closed).