Use is_default_field() everywhere instead of comparing to 1

[slaFFik]

xProfile field Name is default, has an id=1 and in different places we can see both:

1. comparison to 1
2. is_default_field() check

I propose to switch to using the is_default_field() and add a filter there as well.

Related and inspired by #6955.

[slaFFik]

I will prepare a patch for this task.

[slaFFik]

There are some places, like xprofile_clear_fullname_cache_on_profile_field_edit(), where we need to stuck with 1, because there is a hard-coded reference to a fullname field value. So it's not a general use default field.
Other proposed changes are in a patch.

But after creating a patch I got the idea, that we actually have can_delete & is_required separately, so it's already possible to fine-tune fields behaviour. So right now I'm not 100% sure that this patch change is required.

Please, advise.

[boonebgorges]

I agree that we should not be hardcoding this, and I think it's an OK idea to have a wrapper function for this purpose.

But there is a long and awful history here, so I want to be careful not to cause more problems. See #3725 (especially [5778]) and [7812]. We should probably be consolidating on the use of bp_xprofile_fullname_field_id() rather than 1.

@slaFFik Can you chew on those tickets and changesets and see what you think?

[r-a-y]

Related: #7109.