Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40543
Bug introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15192

* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request

Part of #32878

Adds e2e tests

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40671

See
https://gitlab.com/gitlab-org/gitlab-ce/blob/f7254a4060b30e3134c6cf932eaba0fc8e249e9a/app/controllers/sessions_controller.rb#L42
for an example of where we set `flash[:notice] = nil`

Before the we would try to `POST` to the project path, which would
result in a 404, because that `POST` is not supported.

By changing this back to a normal link, not not handled in JS, we can
immeadiatly redirect.

Don't add disabled class to loading button

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38962

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.