Fix base button loading style spacing bug

gl-mr-2 is part of GitLab UI utility-class library which is not supported on GitLab yet. We can consider this problem as a bug in the button component. We can address it by adding a class selector for the loading spinner and applying. the gl-mr-2 utility-mixin.

• Related to recent update: #735 (closed)

ToDo (gitlab!30657 (comment 333200150))

• Replace the gl-mr-2 class selector in button.vue with gl-button-loading-indicator class selector.
• Create a gl-button-loading-indicator class selector in button.scss and apply the gl-mr-2 utility-mixin.
• Submit a MR in GitLab UI following the commit conventions.