Always display guest limit input if guestsEnabled

[dunkOnIT]

This one is a bit of a long story.

• For some competitions (eg, https://www.worldcubeassociation.org/competitions/BattleforBelgrade2025/register), the user can't enter any guest amount > 0
• This is because organizers can only change the guest limit in 1/3 cases where guestsEnabled is true in the competition form - restricted ("Guests are only permitted as guests of competitors"). In the other cases (free and unclear) we don't display the guest limit setting at all - despite the organizer having selected "Ask about guests"
• This leads to a situation where a competitor is asked how many guests they want to bring, but isn't able to set a non-zero value for this field (as the organizer can't set a non-zero limit)

I believe this is an inconsistency in the Competition Form, and we should instead always display the guest limit if the organizer selects "Ask about guests".

There are two levels of solution here:

1. **Fix for new competitions: ** Always show the guest limit if "Ask about guests" is selected
2. **Fix for existing competitions: ** We'll still have the issue of existing competitions asking about guests where no non-zero value is allowed. To resolve this, I think we have a few options:
3. Consider a guest_limit of 0 equivalent to NULL if guests_enabled is true
   1. If we're not going to consider guest_limit == 0 as NULL, then we should change the competition form to allow a user to leave a numeric field blank (currently it can start off blank, but they can't replace the 0 once they've put any number in there - ie, an organizer couldn't remove a guest limit once it is set)
4. Write a SQL query that sets all guest_limit to NULL for all open comps with guests_enabled == true AND guest_limit == 0

[gregorbg]

Huh. Need to ponder about this for a bit.