Change layout of system notes when changing title of the issue

Original description/proposal

Description

When we change the title of the issue, we create a system note for that. However this message has two problems:

• the layout is weird (see below)
• the visual importance is way too strong and draws the eyes too much

See https://gitlab.com/gitlab-org/gitlab-ce/issues/25872 for an exemple

Proposal

Enhance the system note about changing title so it takes way less visual space and doesn't draw the attention too much.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/19797

• Modified the layout.
• Used the Bold font and darker text color, and lightened the background color to indicate the text has been changed.
• If we get the system note icons back, there will be an Edit icon for editing title.

cc @awhildy

Current implementation

Proposed design

• Username should be on the same line as the rest of the system message
• Use font-weight: 600 instead of <strong> for original title/new title
• Text color of titles should be #2e2e2e -> $gl-text-color