Automatically add the appropriate `rel` attribute when using `GlButton` with `href`

When using GlButton with an href attribute (i.e. <gl-button href="...">), the developer needs to remember to add a rel attribute (if appropriate):

<gl-button href="https://example.com" rel="noopener noreferrer">Click me</gl-button>

We should update GlButton to automatically apply the appropriate rel attribute, similar to what GlLink does. This would save developers from having to remember to add the rel attribute manually and enforce a consistent usage of rel across our product.

Based on an internal conversation with @ealcantara (https://gitlab.slack.com/archives/C0GQHHPGW/p1588601058335600?thread_ts=1588600819.335400&cid=C0GQHHPGW), this should relatively simple by including the rel_mixin in button.vue.