Add filter to be able to customize the name that displays on the certificate

[donnapep]

Is your feature request related to a problem? Please describe

It's currently not possible to customize the display of the learner's name that appears on the certificate. It displays as First Name Last Name if both are specified, or otherwise uses the learner's display name.

Describe the solution you'd like

Add a filter so that the learner name can be customized on both the certificate template and the certificate.

[yscik]

Could be worth merging #94 for this.

[cena]

Not sure if completely related, but user wants to

add [users'] full name when they finish the course, as our courses are free without any login or any profile

https://wordpress.org/support/topic/users-full-name-can-add-by-them-self-in-no-login-course/#post-14828344

[m1r0]

I agree with @yscik. We should consider solving this in a more general way and not only for one data type. Also, this approach most likely will solve these issues: #285, #284, #134, #116, #103, #102, #93.

Thoughts?

cc @yscik @donnapep

[yscik]

Yeah let's do it! I looked at it briefly before and there were quite a few merge conflicts IIRC — maybe rebasing that branch onto master is easier since it'd be just one pass. Also basically the whole thing is duplicated and exists at two places in the code, could be a good opportunity to clean it up :)

[donnapep]

@m1r0 and I talked and he's planning to review #94, resolve conflicts and make any changes, if necessary.

[StefMattana]

Somehow related to this, mentioned in 5417376-zen:

Although I have asked learners to sign in with the name they want on their certificate, they are all using their user name which is not their proper name. If there was a separate way to sign in as a learner only, that would help both the certificate and website user issues.