GlSkeletonLoader should inherit attributes, at least classes.

Problem

The GlSkeletonLoader uses the createElement function. This does not automatically merge or inherit attributes.

This makes it impossible to apply custom classes:

<gl-skeleton-loader class="my-custom-class" />
<gl-skeleton-loader :class="{ my-custom-class: true }" />
<gl-skeleton-loader :class="['my-custom-class']" />

It also does not work when trying to add a ref:

<gl-skeleton-loader ref="my-element" />

Goal of this issue is to inherit at least the class attribute, if not all attributes.

Possible side effects

Some usages of the GlSkeletonLoader, which include attributes to pass classes, currently exist in the GitLab monolith. There is a potential risk of side effects when releasing this change. The classes set there would then take effect. This needs testing.