Projects gained a field for the MR source branch removal default.
MRs gained fields for whether there are blocking discussions or
conflicts, and for the squash commit used to integrate the MR.

`PipelineBasic` gained fields for creation/update dates.

Projects gained a field indicating whether they have an empty repo.
Issues gained a field indicating whether there is an empty task list.

Issues and MRs also gained a field for task completion, but we leave
that unimplemented for now.

The merge requests field for multiple assignees now includes an entry
for a single assignee.  Namespaces of projects gained fields for a URL
and an avatar.

Users gained a "highest_role" field when queried as an admin.

Projects and Merge Requests also gained some fields.  Add samples
to our test data but leave out the implementation for now.

Teach `data/fetch_from_gitlab.py` to save the JSON documents with
human-readable formatting and in sorted order.  Use `jq -S .` to format
our existing data the same way.  Formatted JSON documents are still
sufficient to test our crate's deserialization of them and will make
updates to new GitLab versions much more readable.

While at it, also teach the script to remove the user identities,
which we previously did by hand on each update.

Issues gained a field for the number of merge requests referencing them.
Merge requests gained a field for the current user's access.

Merge requests gained fields to report a merge error and whether a
rebase is in progress.

* The `Master` access level has been renamed to `Maintainer`.

* A `cannot_be_merged_recheck` merge status was added.

* Merge requests gained `squash` and `allow_collaboration` booleans.
  The latter supersedes `allow_maintainer_to_push` which is now deprecated.

* Projects gained a `readme_url`.

Notes gained fields describing their resolution.
Merge requests gained a field describing its diff.

Merge requests gained fields for users that merged or closed them
and for whether or not a maintainer can push.

Project hooks gained fields for confidential events.

Merge requests gained a `changes_count` field indicating the number
of paths changed by the MR.  This is the number of entries in the
"Changes" tab of the MR page.  Somehow GitLab reports this as a
string even though it is a number, so parse it ourselves.

Issues and merge requests gained `discussion_locked` fields.
Issues gained a `closed_at` field.
Repo commit detail gained a `last_pipeline` field.

Issues and merge requests gained `time_stats` fields.
Projects gained a `resolve_outdated_diff_discussions` field.
Users restored the `theme_id` field, but it is now optional.

Fetch api v4 sample data from a GitLab 9.2 instance.  Using that and
GitLab sources `doc/api/v3_to_v4.md` and `lib/api/entities.rb`, update
our types accordingly.  Most changes are to rename "builds" to "jobs" or
"pipelines".

Also add strong types to internal ids in preparation for switching to
them when constructing API endpoints.