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
In the case of misuse or misunderstanding from a developer whereby,
if `sel` can receive user-controlled data, jQuery `$(sel)` can lead to the
creation of a new element. Current usage is using hard-coded selectors
in the templates, but nobody prevents that from expanding to
user-controlled somehow.
