Fix "Host is invalid" error when TLD >10 chars (#7948)

Related to https://meta.discourse.org/t/host-is-invalid-error-when-tld-is-longer-than-7-characters/46081.

Using Discourse `v2.4.0.beta2 +119`, I can't add an host (when embedding, cf. `/admin/customize/embedding`) ending with `.engineering`.
Turns out current regex limits to 10 characters.

Fix is dumb: it only allows for up to 24 chars, which is the **current** max TLD length, see https://stackoverflow.com/a/22038535/1907212.

---

Maybe a better (and longer-term) fix would be to allow for up to 64 chars, which I understand comes from the RFC.
I'm not at ease with regexes, so can't be sure about it, but [this suggestion](https://meta.discourse.org/t/host-is-invalid-error-when-tld-is-longer-than-7-characters/46081/8?u=julienma) seems pretty good:

> rules of DNS labels are:
>
> - All labels are 1 to 63 characters, case insensitive A to Z, 0 to 9 and - (hyphen), all from ASCII.
> - No labels may start with a hyphen.
> - No top level domain label may start with a number.
>
>That means a regexp for a valid domain name would look like:
>
>`/^([a-z0-9][a-z0-9-]{0,62}\.)+[a-z][a-z0-9-]{0,62}\.?$/`
>
>Domains that are just a TLD are sufficiently bizarre as to be worth ignoring.

app/models/embeddable_host.rb

@@ -63,7 +63,7 @@ class EmbeddableHost < ActiveRecord::Base
   end
 
   def host_must_be_valid
-    if host !~ /\A[a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,10}(:[0-9]{1,5})?(\/.*)?\Z/i &&
+    if host !~ /\A[a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,24}(:[0-9]{1,5})?(\/.*)?\Z/i &&
        host !~ /\A(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})(:[0-9]{1,5})?(\/.*)?\Z/ &&
        host !~ /\A([a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.)?localhost(\:[0-9]{1,5})?(\/.*)?\Z/i
       errors.add(:host, I18n.t('errors.messages.invalid'))