[meta] Review text link styling throughout GitLab

Resources

FE @alfredo1

This will be a meta issue for the review of anchors throughout the app. These issues will be focused on small actionable changes that allow us to easily improve the current experience. Let's discuss bigger changes on different issues.

Tasks:

• gitlab-ce#24125: Make sure that all anchors share the same style on hover and focus
• gitlab-ce#24129: Add underline for all anchors on hover and focus
• gitlab-ce#24130: Add color change on hover for all anchors

Some points that we should consider when we perform a bigger redesign:

• Find a better way to convey focus state, as the Bootstrap glow is not perfect.

cc @awhildy @tauriedavis @dimitrieh @pedroms @hazelyang