Limit number of issues that can be linked to an issue

We are currently limiting the number of epics that can be related (linked) to another epic to a maximum of 100. We'll be using the same value for work item links &7459 (comment 1509741722) but we don't have a limit in place for linked issues.

There are existing issues that surpass this number of links (~1600) but they would remain valid because the limit validation would only apply to new links.

It was agreed on this discussion that a limit of 100 would be appropriate for issue links too.