Resolves #300 - Set missing participant value on FSDB import

[jsastreh]

Populate "glider_cert" field with "class" FsCustomAttribute when importing pilots from FSDB file.

[biuti]

Id like to populate 'glider_cert' only if 'class' value is a valid certification.
Sometimes they are 'Standard" or 'Sport' or other classes like that.
In these cases it creates a CustomAttribute called 'class'.
I'd also like a neat result with only 'glider_cert' or 'class' CustomAttribute populated.

You get valid certification values using

FAI-Airscore/airscore/core/frontendUtils.py

Line 2283 in ac15c12

def get_certifications_details() -> dict:

And then checking against values of the comp_class, like it is done here:

FAI-Airscore/airscore/core/pilot/participant.py

Line 242 in ac15c12

classes_are_certs = all(c in certs for c in classes)

But accepted certifications list should be calculated in fsdb.py once to avoid reiterating for each Participant.
In Excel import procedure it iterates all participants and switches from CustomAttribute to 'glider_cert' only if all column values are valid.
I'm not sure this is feasible in lxml.
Best solution that comes to my mind at the moment would be to let them be imported as CustomAttribute, then check all Participants against the valid certification list, and if confirmed, copy values to 'gliders_cert and delete CustomAttribute before saving in database.