Fix Live Markdown Preview in personal and subgroup projects

What does this MR do?

This MR addresses #339117 (closed) by fixing how we get the preview markdown path in the FE. Instead of building this URL in the FE, we get it from the BE now 🎉

As a side-effect, we also fix the feature when GitLab installed under a relative URL root

Screenshots or Screencasts (strongly suggested)

Description	Screenshot
✅ Single File Editor - Personal Project
✅ Single File Editor - Subgroup
✅ Web IDE

How to setup and validate locally (strongly suggested)

To test in the Single File editor:

1. Go to a project that has a Markdown file (with personal namespace or subgroup namespace)
2. Click the Markdown file to view it
3. Click "Edit"
4. Right click in the editor and hit "Preview Markdown"

To test in the Web IDE:

1. Go to a project that has a Markdown file (with personal namespace or subgroup namespace)
2. Click the Markdown file to view it
3. Click "Edit in Web IDE"
4. Right click in the editor and hit "Preview Markdown"

To test in a relative URL root:

1. Change your local GDK's gdk.yml to have a non-empty value under relative_url_root (see docs)
2. Run gdk stop && gdk reconfigure
3. Run gdk start and visit your GitLab instance under the new relative URL

Expected:

• No errors in console
• The markdown preview should be loaded next to the editor

Please note:

• There's some already known issues including (#339053 (closed), #339054 (closed))

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.

Related to #339117 (closed)