This also refactor the email_helper support spec to watch for multiple
emails being sent.

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

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

So we can keep showing it to a user in his project page.

When no fork network exists for the source projects, we create a new
one with the correct source

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.

- introduce state machine
- use before transitions to clear data
- introduce namespace validation

Closes gitaly#601

Before this change, the MR counter in the sidebar would be wrong if an MR had
been merged since the last update, but not opened or closed, as merging did not
trigger a counter cache update.