Load issuable participants asynchronously

The database query for Issue and Merge Request participants is pretty heavy for such a small piece of content that users generally won't need instant access to. We could improve page load performance on Issues and Merge Requests by loading the content asynchronously instead.

from: @jschatz1

Design

• Simply use the font awesome spinner to indicate that we are we are waiting for the button / button state.

Out of scope

• We are not doing the below design, because of https://gitlab.com/gitlab-org/gitlab-ce/issues/23206#note_43355199

• The button with the spinner indicates there is a button which is syncing.

• When the syncing is finished, the button will expand to be the button with the text.

SVG: Spinner