Validate urls like http://.com or http://..com

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11702
| License       | MIT

This PR fixes the regular expression to accept valid URLs. It does not allow to start a domain name with "-" or "."
Allowing only *toplabels* in URLs was introduced on purpose in #10586 and this makes sense since it [is valid according to the RFCs](https://github.com/symfony/symfony/issues/3207#issuecomment-32186294). You can, for example, set these hostnames inside your local hosts file or in DNS server used in a local network. Thus, URLs with hostnames like *co* or *com* must be accepted. Even *c* is to be accepted (but that didn't break in this PR, it wasn't accepted before).