We had a couple of form fields that were common to all CI configuration

forms, and have another one coming shortly. To avoid continuing to

duplicate fields, and to ease the development of new CI integrations,

this change introduces a new BaseCIIntegrationConfigForm class that

can house these fields.
All CI integrations now use this form as a base. The Travis-CI

integration does need to override the conditions field in order to

limit to GitHub repositories, but inherits everything else.

Unit tests pass.
Tested each CI configuration form.