quick fix for #614 (no webview on old devices)

 * fixes #614 by invoking the browser intent on old devices again
 * also the alert dialog warning will be shown
 * new option to disable this dialog

## Jenkins testruns
### Multi-Job
https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/210/
### Changed WebViewActivityTest
Both tests of WebViewActivityTest are made for Jenkins Multi-Job to test them on the correct devices. Forced to test it either on emulator or device will fail on those tests where the API does not match.
https://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-device/32/
https://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-emulator/148/