Add feature to be able to upload a file to an issue

Created by: tudorpavel

The idea of this feature is to have the possibility of uploading a file to an issue, not just to comments. This implementation works just for the simpler case of only one file per issue.

Feature requests:

• http://feedback.gitlab.com/forums/176466-general/suggestions/4501700-allow-image-attachments-when-creating-new-issues
• http://feedback.gitlab.com/forums/176466-general/suggestions/4095092-allow-file-uploads-to-an-issue

User Interface

The implementation consists of:

• a slightly modified add/edit issue form
• a slightly modified show issue page

Add/Edit Form

Before:

After, without any attachment:

After, with image attachment:

After, with non-image file attachment:

Show page

Before:

After, with image attachment:

After, with non-image file attachment:

Implementation

• migration to add attachment column to the issues table
• modify Issue model to add CarrierWave attachment the same way it's done for Note
• add issue :delete_attachment route
• add attachment code and style in views so it looks and works very similar to the notes attachment
• had to generalize FilesController#download to work with any model