Add viewer URL to StartupJS

What does this MR do?

This adds the viewer URL to StartupJS

Screenshots

In these examples, I viewed a blob called package.json

Before

The blob content was loaded towards the end of the timeline.

After

The blob content was loaded towards the start of the timeline.

Does this MR meet the acceptance criteria?

Conformity

• I have not included a changelog entry, because this is a developer-facing change.
• I have not added/updated documentation, because this is a developer-facing change.
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)

Related to #330246 (closed)