Refactoring of the GORM model for shares

[glpatcern]

With this PR we introduce new constraints and rename some fields for better consistency:

• Types used by OCM structures only are prefixed with
  Ocm, and AccessMethod and Protocol were
  consolidated into OcmProtocol
• ItemType is used in OCM shares as well
• The (FileIdPrefix, ItemSource) tuple is now
  (Instance, Inode) in OcmShare, and it was
  removed from OcmReceivedShare as unused
• Unique index constraints have been created for regular Shares
  and for OcmShares on (instance, inode, shareWith, deletedAt)
• The unique indexes have been renamed with a u_
  prefix for consistency: this affected u_shareid_user,
  u_link_token. The i_share_with was dropped
  as redundant.
• Alias and Hidden were added in OcmReceivedShare

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[rawe0]

I've tested this version and it doesn't seem to break anything in the normal shares/public links - the DB is fine without migration, however the OCM tables need to be recreated because of the rename from access methods -> protocols and the fileidprefix rename.

[glpatcern]

As we've seen with Rasmus, everything works but the indexes are not created the way we want, so this requires a bit more work before going ahead.

[glpatcern]

Now tested in full on a dev node - I created all types of shares and everything worked.