activity screen - do not show blank slate when search is active AGPUSH-1500

[lfryc]

https://issues.jboss.org/browse/AGPUSH-1500

I've introduced a helper isSearchActive() that is now a condition for showing blank slate.

Besides that I've fixed the Register Device button also on Activity page (was fixed before on Analytics: https://github.com/aerogear/aerogear-unifiedpush-server/blob/1.1.x-dev/admin-ui/app/components/app-detail/include/analytics.html#L97 )

🎯 master and 1.1.x-dev
🚢 deployed at https://upsstaging-aerogearz.rhcloud.com/ag-push/ (standard credentials)
🔨 Testing:

• try Activity tab with no devices registered (blank slate should be shown)
• try Activity tab with one device registered (blank slate should be shown)
• try Activity tab with a message sent (the activity log table should appear)
• try Activity tab with a message sent, but use search with a non-existent value (activity log table should be visible, but a message "No message matches the given search criteria." will appear - then try to clear search string and a message should appear again)

[lfryc]

This PR replaces #675

[lfryc]

1.1.x-dev: 0bd6f62

master: 173abf0

[lfryc]

Thanks @sebastienblanc !