Add an expect call to prevent random Spinach failure

This should help prevent getting the error:

```
Unable to find field "merge_request_title
```

This was happening probably because it takes some time after the "Compare branches" button is hit before the "New merge request" page showed up.

See merge request !882

Fix consistency issue in sidebars of Project and Group Settings.

Fixes #2277

![before_after_group_settings_nav](https://dev.gitlab.org/gitlab/gitlabhq/uploads/370c701d0eff8f1f8e44cf954c75f2bf/before_after_group_settings_nav.png)

See merge request !1871

Attempt to fix the "I should see that I am [un]subscribed" steps

Makes use of Capybara methods that wait. Hopefully this will fix the
random timing-related failures of this step.

See merge request !853

Reorder MergeRequestTabs constructor

The tab was being activated before we set the `[commits|diffs]Loaded`
variable, so even when the `/diffs` route is accessed directly, like
from the "Side-by-side" link, the tab was being loaded from its default
source.

Fixes #1887

See merge request !892

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Makes use of Capybara methods that wait. Hopefully this will fix the
random timing-related failures of this step.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>