[Feature] [API] Check If user is subscribed to specific Issue

[6543]

similar to https://gitea.com/api/swagger#/repository/userCurrentCheckSubscription
but for a single issue

Gitea Version: 1.12.0+dev-72-g2c25e75dc

[6543]

• GH has no direct API endpoint
• GET ​/repos​/{owner}​/{repo}​/issues​/{index}​/subscriptions​ already taken (return list of subscribers)

Proposal:

-> GET ​/repos​/{owner}​/{repo}​/issues​/{index}​/subscriptions​/{user} -> WatchInfo

[6543]

cange api to GET ​/repos​/{owner}​/{repo}​/issues​/{index}​/subscriptions​/check because of privacy