_CONF_CHECK_ISSUE_UPDATE_CONFLICT feature leads to heavy load with multiple tabs opened

[paulbutenko]

When user opens lot of tabs with issues, each of them does request every 5 sec https://github.com/Ilogeek/redmine_issue_dynamic_edit/blob/master/assets/javascripts/issue_dynamic_edit.js#L264 even if tab is inactive.

for multiple users with multiple tabs these requests does lot of pressure to redmine.

might be improved by disabling request when tab is inactive.