New endpoints are:

POST :project_id/(issues|merge_requests)/(:issue_id|:merge_request_id)/time_estimate"

POST :project_id/(issues|merge_requests)/(:issue_id|:merge_request_id)/reset_time_estimate"

POST :project_id/(issues|merge_requests)/(:issue_id|:merge_request_id)/add_spent_time"

POST :project_id/(issues|merge_requests)/(:issue_id|:merge_request_id)/reset_spent_time"

GET  :project_id/(issues|merge_requests)/(:issue_id|:merge_request_id)/time_stats"

- Use IssuesFinder for the /issues API resouce
- Tests for iid filter in project issues API resource
- Tests for No Milestone filter in issues API resources
  The "No Milestone" case was not previously tested, and the `/issues`
  resource did not support the the `milestone` parameter.
- Return issues where all labels match from the issues and project issues
  API resources, like the group issues resource already does. See
  https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6825#note_17474533

Signed-off-by: Rémy Coutable <remy@rymai.me>

Current filtering for "/projects/:id/issues" endpoint
returns no results for "No Milestone" query.

This PR introduces fix by copying filtering from "/groups/:id/issues"
which works as expected.

Following a discussion in !7180

When a merge request can only be merged when all discussions are
resolved. This feature allows to easily delegate those discussions to a
new issue, while marking them as resolved in the merge request.

The user is presented with a new issue, prepared with mentions of all
unresolved discussions, including the first unresolved note of the
discussion, time and link to the note.

When the issue is created, the discussions in the merge request will get
a system note directing the user to the newly created issue.