Fix/open api fixes

[seferturan]

⚠️ Warning ⚠️

• Contains breaking changes.

🎶 Notes 🎶

• Fixes Openapi kotlin generator results are incorrect #720, fixes [kotlin] checkin paths are the same, but have separate routes #721, fixes [kotlin] Watching responses need to be nullish #722, fixes [kotlin] Collection responses need to be nullish #723, fixes [kotlin] Liked list responses need to be nullish #724
• Address some issues that resulted in the open api kotlin generator to generate wrong results.
  • The checkin paths are the same, but used different routes. This is now merged into a start route.
  • Makes some more responses nullish since the generator can't deal with discriminated unions.
• The generated openapi.json was tested by @michaldrabik

[michaldrabik]

@seferturan Please also make these connections boolean fields for user setting nullish while at it:

GET /users/settings

@Serializable
data class GetUsersSettings200ResponseConnections(

    @SerialName(value = "facebook")
    val facebook: kotlin.Boolean,

    @SerialName(value = "twitter")
    val twitter: kotlin.Boolean,

    @SerialName(value = "mastodon")
    val mastodon: kotlin.Boolean,

.......