Add 'unique' option to 'select' component

Problem/Motivation

Currently, it's not possible to enforce unique values for select components as it is with other components, such as email. For example, with the email component, it's possible to restrict submissions such that a single e-mail address can only be submitted once for a given webform. This issue proposes extending the select component to support the same functionality so that once a select option has been submitted, it is no longer available for future submissions.

Proposed resolution

• Add a 'Unique' checkbox as an option on the 'select' component's edit page
• Extend _webform_render_select() to run submissions through webform_validate_unique() if unique is selected for the component
• Extend _webform_render_select() to remove previously submitted options from the options array if unique is select for the component
• It will be possible to disable options instead of removing them once #1077532: Implement "Disabled"-functionality for "Select options" also is committed
• If no options remain after previously submitted options are removed, then the component is disabled and an empty option labeled, "No options available" is printed

Remaining tasks

Need to submit patch

User interface changes

'Unique' checkbox is added as an option for the 'select' component on the component's edit page.

API changes

None

Data model changes

None