Reusing `/api/v1` from Gitea UI Pages have pros and cons.
Pros:
1) Less code copy
Cons:
1) API/v1 have to support shared session with page requests.
2) You need to consider for each other when you want to change something about api/v1 or page.
This PR moves all dependencies to API/v1 from UI Pages.
Partially replace #16052
* introduce GET /notifications/new
* add TEST
* use Sprintf instead of path.Join
* Error more verbose
* return number of notifications if unreaded exist
* 200 http status for available notifications
