Warn users when redirection happens

This comes from: https://github.com/owncloud/android/pull/3074#issuecomment-789682250

If a redirection happens, user must be notified and approve explicitly the issue (a similar way to do as in a self-signed certificate)

iOS example:

AC:

• When the URL is checked, if a redirection happens, a dialog/alert/whatever is displayed to notify the issue
• When the user accepts, auth flow goes on
• In case the user does not accept the redirection, the auth flow stops

NOTE: this is the same way to do as in a self-signed/not secured server certificate

TASKS

• Research (if needed)
• Create branch feature/feature_name
• Development tasks
  • Create new exception when redirection 301 or 302 happens
  • ...
• Code review and apply changes requested
• Design test plan
• QA
• Merge branch feature/feature_name into master

PR

• App
• Library (if needed)

The issue names no files or tests; start by tracing URL checking and the authentication flow, then compare the existing self-signed or unsecured certificate handling. Done means a 301 or 302 redirect shows an approval prompt, continues authentication after acceptance, and stops the flow when declined.