Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38019

The ApplicationSetting model uses the CacheMarkdownField concern, which updates
the cached HTML when the field is updated in the database. However, in specs,
when we want to test conditions using ApplicationSetting, we stub it, because
this is accessed in different ways throughout the application.

This means that if a spec runs that caches one of the Markdown fields, and a
later spec uses `stub_application_setting` to set the raw value of that field,
the cached value was still the original one. We can work around this by ignoring
the Markdown cache in contexts where we're using `stub_application_setting`.

We could be smarter, and only do this on the Markdown fields of the model, but
this is probably fine.

I've followed the [upgrade guide](https://github.com/thoughtbot/factory_bot/blob/4-9-0-stable/UPGRADE_FROM_FACTORY_GIRL.md) and ran these two commands:

```
grep -e FactoryGirl **/*.rake **/*.rb -s -l | xargs sed -i "" "s|FactoryGirl|FactoryBot|"
grep -e factory_girl **/*.rake **/*.rb -s -l | xargs sed -i "" "s|factory_girl|factory_bot|"
```
Signed-off-by: NRémy Coutable <remy@rymai.me>

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

(10.2) Avoid partial partial email adresses for matching

See merge request gitlab/gitlabhq!2232

(cherry picked from commit 081aa1e91a777c9acb31be4a1e76b3dd7032fa9a)

There are unresolved conflicts in app/models/user.rb.

fa85a3fd Don't allow searching for partial user emails

Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.

To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.

Because the checks are now done outside normal request, we can have a
simpler failure strategy:

The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.

When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.

After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.

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