This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

We can't properly use foreign keys on columns that are configured for
polymorphic associations which has disadvantages related to data
integrity and storage. Given we only use time tracking for Issues and
Merge Requests we're moving to the usage of regular associations.

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"