Add SFW content mode option

[WithoutPants]

Adds a SFW content mode option, found in Settings -> Interface:

Also adds an option to set the mode in the setup wizard:

When this option is enabled, the following changes are made:

• the following porn-specific performer fields are hidden:
  • death date
  • ethnicity
  • height
  • weight
  • hair color
  • eye color
  • measurements
  • penis length
  • fake tits
  • circumcised
• o-count is renamed to likes and uses the thumb up icon
• default performer image is replaced with a generic user icon - the same used for the performers navbar icon

Also adjusts the wording in the README to reflect that stash supports non-porn-related content.

Screenshots:

[smith113-p]

That list of performer attributes seems too big to me. For example, IMDb has birthday, death date, height for this definitely not porn performer:
https://www.imdb.com/name/nm0000026/bio/