Fix checkbox alignment in application settings

Add the form-control CSS class to the feature checkboxes on the application settings page to fix the vertical alignment with their labels.  See #2053.

Also add aria-describedby attributes to form controls that have a help text block (http://getbootstrap.com/css/#callout-help-text-accessibility).

## Before

![Admin_area___GitLab](https://dev.gitlab.org/Okada/gitlabhq/uploads/2736fe81ba564b43cbef37cd187cf47a/Admin_area___GitLab.png)

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## After

![Admin_area___GitLab_-_after](https://dev.gitlab.org/Okada/gitlabhq/uploads/95a8a26600458c9c55bc05d2e0c6be47/Admin_area___GitLab_-_after.png)

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This doesn't address the help text block for the twitter checkbox.  @dzaporozhets Do you want to keep the help text visible or would you rather put it in a dialog and hide it initially?

cc @sytse

See merge request !1615