Validate user input URLs on redirect form

[humitos]

There are cases where allowing the user to input a To URL without a leading slash produce weird redirects. See #4501 (comment)

Besides the case described in that comment, there are other that needs to be validated. For example when we redirect to external sites, this should allow a URL that starts with http for example.

I pushed a commit for one of these cases at f6fed2d but I also realize that we are using jQuery to dynamically change the inputs of the form. This is a problem when the form returned a validation error since it starts with the default state (no inputs at all) and a red error message which could confuse even more our users.

[stsewd]

Related to https://github.com/rtfd/readthedocs.org/issues/4023

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stsewd]

Should we allow URLs without / at the beginning? I mean, for no-external urls.