Add algorithm to generate Level3URL automatically (for Juicebox Pro galleries)

[vdhamer]

Fotogroep Waalre's originally loaded level2 membership data using an HTML table instead of fgWaalre.level2.json.

The level2.json file in turn contains - per member - a level3.json URL used to access the portfolio, and a featured image/thumbnail that is shown in the Portfolios screen. This works, but requires filling in level3URL and featuredImage and featuredThumbnail for every single club member.

This can optionally be simplified by providing club-level information (in the header of the level2.json file) which indicates the base URL to use for portfolio and featuredImage/Thumb.
So, if this level3BaseURL is available, creation of level2 files is simplified: just provide a list of members, the level3BaseURL, and an algorithm generates the full level3URL per user.

Benefits:

• promotes consistency across clubs (which is always good for software)
• level2.json club files that use this feature don't need to manually provide level3URL strings
• partial automation of level3URL names prevents certain errors
  • e.g. " " becomes "_"
  • special characters in a member's name are replaced (e.g. "é" becomes "e")
• also serves to automatically extract a featuredImage/featuredThumbnail

Subtasks:

• add CoreData property level3BaseURL to the CoreData Organization entity
• add level3BaseURL OrganizationOptionalFields.swift
• add JSON level1 parsing for level3BaseURL to update value stored in CoreData
• add JSON level2 parsing for level3BaseURL to update value stored in CoreData
• fill level3URL programmatically if level3URL is empty and level3BaseURL present
• fill featuredImage programmatically if image URL is empty and level3BaseURL present