Fix DB Compatibility & small fixes

[domrim]

I tried to use Pønskelisten with a postgresql database. It didn't work so i fixed the code so it works now. mysql/mariadb should still work perfectly.

also found some redundant code and small bugs which i fixed :)

[aunefyren]

Wow, thank you so much for the contribution. I may take me some time to look through this code before merging, but it looks really good! I have learnt a lot as this project evolved so I appreciate the clean up and hope the code wasn't too hard to decipher.

I know my previous Gorm approach didn't support Postgres, but I learned as I went. Your approach is completely new to me so I will have to learn it for future development. This is preferable though, as I want to support more DB formats. Seems to be this approach: https://gorm.io/docs/advanced_query.html

[aunefyren]

Still looking through and debugging on the MariaDB side. Your changes didn't work out of the box. I have fixed some issues while (hopefully) maintaining PostGresSQL support.

Now I am stuck at this issue:

groupRecords := Instance.Distinct().
		Debug().
		Group("groups.id").
		Where(&models.Group{Enabled: true}).
		Joins("JOIN `group_memberships` ON `group_memberships`.`group_id` = `groups`.`id`", Instance.Where(&models.GroupMembership{Enabled: true, MemberID: UserID})).
		Find(&groups)

That code produces this MYSQL statement:

SELECT DISTINCT `groups`.`id`,`groups`.`created_at`,`groups`.`updated_at`,`groups`.`deleted_at`,`groups`.`name`,`groups`.`description`,`groups`.`enabled`,`groups`.`owner_id` FROM `groups` JOIN `group_memberships` ON `group_memberships`.`group_id` = `groups`.`id` WHERE `groups`.`enabled` = true AND `groups`.`deleted_at` IS NULL GROUP BY `groups`.`id``

Which does not include the WHERE restrains on GroupMemberships. Not sure why.

Honestly I am considering scrapping MariaDB support altogether and migrating to PostGresSQL.

[domrim]

i rebased my branch on yours to include the few fixes of the last month ;)