Add JS prefix to user tab classes
## What does this MR do
This MR adds the `js-` prefix to the classes which are used for js purposes only. This MR only affects the said classes inside `users/show.html.haml`.
## Are there points in the code the reviewer needs to double check?
I'm not entirely sure if this project uses the `js-` prefix for
classes that are used for js purposes only. Doing a grep shows some of the classes have the prefix, but I'm still unsure whether this project follows this rule.
## Why was this MR needed?
As I was working thru #13401, I wanted to add a tab on the user profile page to show the contributions outside the GitLab instance. Reading the JS, I came across this. This MR helps new people reading the code that the said classes are for js purposes only. Also, it helps the project work toward to uniform class naming.
I suggest that we document adding the `js-` prefix to classes for js purposes only (if this is the case for this project)
See merge request !3928
a792427eed