Add a preferences API so apps can share basic behaviours

[Gargron]

Resolve #9821

GET /api/v1/preferences example output:

{
  "posting:default:visibility": "public",
  "posting:default:sensitive": false,
  "posting:default:language": null,
  "reading:expand:media": "default",
  "reading:expand:spoilers": false
}

[ykzts]

oops. this PR was draft

[Gargron]

@Tak @connyduck Is this sufficient?

[Tak]

@Gargron Sorry, it's not clear to me from the diff - does this also allow apps to write these preferences?

A minor comment on the output format - if the keys are meant to be hierarchical, why not represent them that way in the json?

"posting": {
  "default": {
    "visibility": "public",
    "sensitive": false,
    "language": null
  }
},
"reading": { //...```

[Tak]

Cool, good discussion.