* processes any pending records which are not migrated yet
* bumps import_export version because of new commits_count attribute
* removes commits_count fallback method

This reverts commit 0b7d1085.

* processes any pending records which are not migrated yet
* bumps import_export version because of new commits_count attribute
* removes commits_count fallback method

The st_commits and st_diffs columns on merge_request_diffs historically held the
YAML-serialised data for a merge request diff, in a variety of formats.

Since 9.5, these have been migrated in the background to two new tables:
merge_request_diff_commits and merge_request_diff_files. That has the advantage
that we can actually query the data (for instance, to find out how many commits
we've stored), and that it can't be in a variety of formats, but must match the
new schema.

This is the final step of that journey, where we drop those columns and remove
all references to them. This is a breaking change to the importer, because we
can no longer import diffs created in the old format, and we cannot guarantee
the export will be in the new format unless it was generated after this commit.

If the import version changes, we need to update a few project templates
to match this new version.

In a seperate commit I'll update the docs to reflect this.

Related to !13108. Mostly this is just running the rake task and
changing the task a bit to catch cases like the project already existing
or so. The rake task moves archives to the vendor/project_template
directory, which are checked in too.

[ci skip]

Started implementation for the first iteration of
gitlab-org/gitlab-ce#32420. This will allow users to select a template
to start with, instead of an empty repository in the project just
created.

Internally this is basically a small extension of the ImportExport
GitLab projects we already support. We just import a certain import
tar archive. This commits includes the first one: Ruby on Rails. In the
future more will be added.