Truncate long filenames

[vasantvohra]

Users occasionally upload a file with a very lengthy name (>250) in the file field of the registration form; Mac and most OS allow up to 255 characters, but our servers return the error [Errno 36] File name too long.

Traceback:

File "/nas/indico.un/v2/indico-un/indico/indico/core/storage/models.py", line 208, in save
    self.storage_file_id, self.md5 = self.storage.save(path, self.content_type, self.filename, data)
  File "/nas/indico.un/v2/indico-un/indico/indico/core/storage/backend.py", line 252, in save
    with open(filepath, 'wb') as f:
StorageError: Could not save "event/1000135/registrations/5552/614104/102009-1633527965-AREMDCAM_AND_AUAG-CAMEROON-LETTER_OF_NOMINATIONS_ASSIGNMENT__RECOMMENDATION_FOR_ACCREDITATION_TO_THE7th_SESSION_OF_THE_MOP_TO_THE_AARHUS_CONVENTION_4TH_SESSION_OF_THE_MOPP_ON_PRTRs_THEIR_JHLS_AND_ASSOCIATED_MEETINGS_18-22_OCTOBER_IN_SWITZERLAN.pdf": 

[Errno 36] File name too long: u'/indico.un/v2/data/archive/event/1000135/registrations/5552/614104/102009-1633527965-AREMDCAM_AND_AUAG-CAMEROON-LETTER_OF_NOMINATIONS_ASSIGNMENT__RECOMMENDATION_FOR_ACCREDITATION_TO_THE7th_SESSION_OF_THE_MOP_TO_THE_AARHUS_CONVENTION_4TH_SESSION_OF_THE_MOPP_ON_PRTRs_THEIR_JHLS_AND_ASSOCIATED_MEETINGS_18-22_OCTOBER_IN_SWITZERLAN.pdf'

[vasantvohra]

May I get a review here? I've tested this PR on v3

[ThiefMaster]

Looks fine, but I'd like to have a unit test for this. I think we have a test for the function itself, so adding a new case covering this might be enough.

[vasantvohra]

Hello, Morning, I added a test case to cover this. But, it is a hardcoded string of length 255, let me know if you'd prefer something autogenerated.