This helps keep the abstraction layers simpler, and also keep the
interface of those methods consistent, in case of implementation
changes.

The raw_log method is meant to become the Gitaly RPC boundary. By
setting the defaults before doing the RPC we keep the RPC
implementation simpler. We also sidestep the unfortunate subtleties of
what happens when options[:limit] is not set, or nil.

Originally picked as: 3fc0dbcb. But
given the internals changed, this was changed now. The changelog was
removed too.

Closes gitaly#433

Closes gitaly#421

Previously, we stored these as serialised fields - `st_{commits,diffs}` - on the
`merge_request_diffs` table. These now have their own tables -
`merge_request_diff_{commits,diffs}` - with a column for each attribute of the
serialised data.

Add a background migration to go through the existing MR diffs and migrate them
to the new format. Ignore any contents that cannot be displayed. Assuming that
we have 5 million rows to migrate, and each batch of 2,500 rows can be
completed in 5 minutes, this will take about 7 days to migrate everything.

This would be much more accurate. We assume this is an
auto-generated email if such header is provided, and
the value is not "no". It could also be: "auto-generated",
"auto-replied", or other values from extension. It seems
that only "no" could mean that this is sent by a human.

See: https://tools.ietf.org/html/rfc3834

Also make it easier to have mailer helper

full_authentication_abilities. This is fine because
we're going to check with can?(..) anyway

Closes gitaly#415

First round of review, main changes:
- templates.title is human readable, #name will be passed around
- GitLabProjectImporterService has been renamed

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

First iteration, and some stuff is missing. But basically this rake task
does a clone of a project we've pointed it to. Than creates a project on
the GDK, which should be running in the background. This project is
exported, after which we move that archive to the location we need it.
We clean up by removing the generated project.

The first idea was to export the project on .com too, however than we
might run into ImportExport versions mismatch. This could've been
circumvented by checkout out an older commit locally. This however is
not needed yet, so we opted to not go this route yet, instead we will
iterate on what we got.