add `since` query string support to list issue comment

* Issues can be closed via API

* Error-checking is nice xD

* EditIssueOption.Status => State

* Use const instead of string-literal

* Add AddCollaborator API Endpoint

* Add optional Permission to AddCollaborator endpoint

* Use APIContext

* Add API support for labels.

* Error handling for adding/replacing multiple issue labels

* Revisions to function names and error handling. Use issue.ClearLabels in replace/clear functions

* Additional code cleanup

Add sign in check before check user account level

Allows the deletion of a webhook from a repository at the
/:user/:repo/hooks/:id endpoint.

Solves drone/drone issue #1603.

Signed-off-by: Dennis Chen <barracks510@gmail.com>

Additional properties: created_at, updated_at

Adds description, stars_count, forks_count, watchers_count and
open_issues_count.

This prevents user retrieve arbitrary webhook by changing URL to
access webhook from other unauthorized repositories.

Add start count corrector for Repository.NumClosedIssues

- Fix go-gogs-client#10
- Related to #809

Both are possible on explore and admin panel