Web IDE markdown visualisator have issue with image path
<!---
Please read this!
Before opening a new issue, make sure to search for keywords in the issues
filtered by the "regression" or "bug" label.
For the Community Edition issue tracker:
- https://gitlab.com/gitlab-org/gitlab-ce/issues?label_name%5B%5D=regression
- https://gitlab.com/gitlab-org/gitlab-ce/issues?label_name%5B%5D=bug
For the Enterprise Edition issue tracker:
- https://gitlab.com/gitlab-org/gitlab-ee/issues?label_name%5B%5D=regression
- https://gitlab.com/gitlab-org/gitlab-ee/issues?label_name%5B%5D=bug
and verify the issue you're about to submit isn't a duplicate.
--->
### Summary
Web IDE markdown visualisator have issue with image path, the path is not relative but absolute.
### Steps to reproduce
1. have a repository with this hieracrchiy:
/doc/mydoc.md
/doc/images/myimage.png
2. into mydoc.md have

3. go to Web IDE > open /doc/mydoc.md > click on preview markdown tab
**Result**:=> the image is not visible
the link to the image is not relative : https://gitlab.server.com/user/repo/master/images/myimage.png
The visualisation work fine we open file into gitlab as:
https://gitlab.server.com/user/repo/blob/master/doc/mydoc.md
issue