Asynchronous loading of an issue's related merge requests
When viewing certain issues (e.g. gitlab-com/operations#42 (closed)) you'll see a list of related merge requests. Loading this data on average takes around half a second (http://performance.gitlab.net/dashboard/db/issue-referenced_merge_requests). While we're working on improving this it may take a while for this to be resolved once and for all. Since this data is not really crucial when working with issues I propose that we load this data asynchronously, this way we can at least shave off half a second of loading time when viewing an issue.
UI wise we'll probably need some kind of spinner. We'll also need a way to load the related merge requests as JSON and use this to build the HTML (unless we start sending HTML fragments via XHR).
cc @jschatz1