App cannot open url received with photos notifications

This is a bug. The BASE_URL has to be added to the url so that it is not relative.

Sentry Issue: THALIAPP-16

Error: Could not open URL '/photos/04-06-2019-borrel-icis/': No Activity found to handle Intent { act=android.intent.action.VIEW dat=/photos/04-06-2019-borrel-icis/ flg=0x10000000 }
    at h(index.android.bundle:19)
    at (index.android.bundle:19)
    at value(index.android.bundle:28)
    at (index.android.bundle:28)
    at value(index.android.bundle:28)
...
(2 additional frame(s) were not displayed)