Issues count in epic does not go higher than 100

Summary

When an epic has more than 100 attached issues, the count will initially show 100. When the user clicks Show more, the number will get updated to the actual number (as long as there are less than 200 issues).

Example epic

&989 (closed)

Possible fixes

As MVC, we can initially show 100+ if the X-Next-Page link header is included in our API response.

As long term solution, we should consider storing the count of attached issues in the epic object itself, so that we can display it directly.

/cc @annabeldunstone