show a message at the log-in screen if inital encryption take place

To inform the user that the first log-in after the encryption was enabled could take some time I added a info text to the login screen. This info text will be shown during the initial encryption.

To test this you need to enable encryption, log out and log in again. After entering your username, password and clicking on "Login" you should see a spinner and this message: "Initial encryption started... This can take some time. Please wait.". It is useful to have a few files in your ownCloud so that the encryption takes some time and you can check the message.

Addresses issue #5844 

cc @PVince81