Remove unnecessary / from avatar url

## What does this MR do?

This MR removes the unecessary / from the avatar url.

## What Use Case does this MR solve?

The // in the url cold lead to avatars not being displayed (reported on IRC)

With this MR the avatar url changes from
http://localhost:3000//uploads/user/avatar/1/gitlab_logo.png
to
http://localhost:3000/uploads/user/avatar/1/gitlab_logo.png

@dblessing Can you take a look?

See merge request !303