Show an error if the privacy policy of trävelling has not been accepted yet

[FriederHannenheim]

Is your feature request related to a problem? Please describe.
When making a new trävelling account without accepting the privacy policy and logging in with this account, no results will be shown when searching for stations.

Describe the solution you'd like
The user should be directed to the trävelling website to accept the privacy policy

Additional context
To reproduce:

• Start trävelldroid for the first time
• press the login button
• create a new account using mastodon
• give oauth permission to trävelldroid

Now trävelldroid shows the home page and the station seach bar but no results when entering text.
To resolve this the user has to go to the trävelling website and accept the privacy policy