fix(sharing): Use link_defaultExpDays for default expiration in web UI

[nfebe]

The link_defaultExpDays config was only used in the backend share validation but never exposed to the frontend, making it impossible to set a default expiration date different from the maximum.

[julien-nc]

Thank you for this PR!

This partially works. The default date is now correct: now + link_defaultExpDays if link_defaultExpDays is set.

But the maximum date is the same as the default in the UI. So even if shareapi_expire_after_n_days is higher than link_defaultExpDays, one cannot set a higher date than the default one.

I think this can be addressed in apps/files_sharing/src/mixins/SharesMixin.js where maxExpirationDateEnforced is defined.

server/apps/files_sharing/src/mixins/SharesMixin.js

Line 151 in f06133b

maxExpirationDateEnforced() {

[julien-nc]

IMO this can be backported to 31, 32 and 33.