Updated docs, refactor import/export code
Fix AvatarUploader path issue
Fix project export upload webhook error

Hooks were run for wikis, but given the internal API wasn't responding
failures happended, as seen on:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18693

This reverts commit bf790c26.

Direct disk access is done through Gitaly now, so the legacy path was
deprecated. This path was used in Gitlab::Shell however. This required
the refactoring in this commit.

Added is the removal of direct path access on the project model, as that
lookup wasn't needed anymore is most cases.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1111

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

This reverts commit 54a575f1, reversing
changes made to c63af942.

Move uploads to object storage

Closes #4163

See merge request gitlab-org/gitlab-ee!3867

+ address small nitpicks

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: Rémy Coutable <remy@rymai.me>

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.

This is used in EE for the storage migration, and we want to use this
in CE as well to be able to migrate projects to hashed_storage.

There are some redundancies in the validation steps, and that is to
preserve current error messages behavior

Also few specs have to be changed in order to fix madness in validation
logic.

Also improves the `create_templates` transient attribute and use
`project.project_feature.update_columns` instead of
`project.project_feature.update_attributes!` since it's faster.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Also, fixing some calls to the :project factory with the :test_repo
trait since this trait is already included in the :project factory.
Signed-off-by: Rémy Coutable <remy@rymai.me>