Repository size limit check is off by an order of magnitude

Shortly after the deploy of RC3 on gitlab.com I encountered this error on a merge request:

We've temporarily disabled the size limit on .com until we can address the underlying issue.

Discussion starts on Slack at https://gitlab.slack.com/archives/infrastructure/p1484666901007987

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754 converted storage statistics to bytes, from megabytes.

However, EE has the project size limit feature, which is stored in megabytes. And when it formats the repository's actual size, it assumes it's in megabytes: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/gitlab/repository_size_error.rb#L58

Two obvious options:

1. Migrate the size limit columns to also be in bytes. This means that on user input we have to multiply by a megabyte, otherwise the limits will be too small.
2. Convert the other way, and make our various actual_size_limit methods divide by a megabyte, and then fix the helper.

@smcgivern I'd go for converting everything to bytes. I don't know why we ever stored data in MB in the first place, but it's silly. Migrating the size limit should be trivial (basically something like update foo set X = X * 1024 * 1024).

@yorickpeterse I agree, under the assumption that we probably update this in fewer places than we read it.

mentioned in issue #1542 (closed)

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/26773

We show the size incorrectly on the repository page.

mentioned in issue gitlab-com/support-forum#1454 (closed)

mentioned in issue gitlab-com/support-forum#1455 (closed)

This problem suggests that parts of this code aren't tested properly, otherwise we would have caught it during the EE test runs.

mentioned in issue gitlab-com/www-gitlab-com#1067 (closed)

assigned to @oswluizf

mentioned in merge request !1067 (merged)

closed via commit 2d5901c47d7a03c4318bc768e67611b3c61d39b3

mentioned in commit c19e4ee4