Requesting new tags is done by `input.php`

We just send the user to the file `input.php`, which will request a random tag.
Massive restructuring

I should’ve been stricter with my commits:
* we can now display errors
* random tag versus specific tag
* handle required fields using JS (to make the skip button work without the user having to fill in things...)
Changed the workings of random tags

Calling `input.php` without a tag turns the code into a redirect to `input.php` *with* a tag that was suggested by the API.