Migrate database to GORM and new schema

[jessegeens]

Following ADR-Reva-003, this PR refactors the sharing SQL logic to now use GORM, an ORM for Go. This change also comes with newly written unit tests for all the database logic.

[diocas]

Some questions and comments. Haven't had the time to review the tests yet.

[diocas]

This might be useful in the migration script, but I would clean it from here.

[diocas]

Enum? Btw, ResourceTypeToItem can return other strings.

[jessegeens]

Real Enum's don't exist in Go, but I can do something like this:

type ItemType string

const (
	ItemTypeFile      ItemType = "file"
	ItemTypeFolder    ItemType = "folder"
	ItemTypeReference ItemType = "reference"
	ItemTypeSymlink   ItemType = "symlink"
)

[jessegeens]

outside of this package you can then do share.ItemTypeFile for example (you can also do illegal values, but then you would have to do ItemType("illegal-value")), so I think this should at least make it clear and kind of enforces it

[diocas]

Can you put the full list of defaults?

[jessegeens]

I'll maybe just remove the "// such as creation_time, id etc" then, you can see the fields by hovering over gorm.Model :)

[diocas]

Remove it then!

[diocas]

I no longer remember what was this FileSource and FileTarget. Maybe it would be worth to have some description for some of these fields. For future reference.

[jessegeens]

FileSource seems to be the same as ItemSource, i.e. the inode (this is true for all but 8 of the entries in the prod db). So this can maybe also be dropped.
FileTarget seems to be the last part of the path of the file, e.g. /MyFile.txt. Since we have the InitialPath now I think we could also drop this

[diocas]

I think we decided to drop those, but I don't see a comment like that in the rfc file you did... Confirm in the code where these are used and drop them if ok.