Use numeric IDs (not paths) as Analytics feature parameters

Explanation

As mentioned in the Analytics API Guidelines Issue description, currently some group::analytics features have group_id and project_id parameters that expect paths instead of integers. For example, group_id=gitlab-org/subgroup. There are some problems with this:

• Unconventional, as groups and projects are specified by ID in most contexts
• Unpredictable, as the parameter name implies the values are IDs
• More complex, as paths require URL escaping

Proposal

For all group::analytics features, the group_id and project_id parameters expect numeric IDs (not paths).