Improve support for long build traces

UX: @dimitrieh FE: @filipa

We recently merged: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10336.

It works great. However, we need a few tweaks by looking at: https://dev.gitlab.org/gitlab/gitlabhq/builds/1000361.

1. Change LIMIT_SIZE to 500 KiB, 50 seems too small,
2. Add to truncate message a link to raw: raw_namespace_project_build_path(@project.namespace, @project, @build),
3. Fix message: currently, we show amount in KiB and we get .size from json which is in bytes. This data should be formatted by the frontend. It seems that it is fine now to divide .size/1024 now,
4. Update amount of data presented when we receive next jsons with size. Currently, our flow is to show the message if !append which is correct. However, we should update truncated size with every received .json,
5. Improve limiting on the backend, to skip to next new line (Stretch).

Design

• making the line opaque (no opacity)
• link to complete trace also in line in build log