Add a relation between IDF_Project and IDF_Tag (again), this time

its a many-to-many. We store project tags in IDF_Tag with a project
id "0" (this has minimal to no impact on existing code) and therefor
only need to ensure that the new relation table exists in the migration.

Then just the project summary configuration and the admin's project
create and project update forms and views needed to be adapted to
be able to render, create and update project tags.

src/IDF/Project.php

@@ -86,6 +86,13 @@ class IDF_Project extends Pluf_Model
                                   'verbose' => __('description'),
                                   'help_text' => __('The description can be extended using the Markdown syntax.'),
                                   ),
+                            'tags' =>
+                            array(
+                                  'type' => 'Pluf_DB_Field_Manytomany',
+                                  'blank' => true,
+                                  'model' => 'IDF_Tag',
+                                  'verbose' => __('labels'),
+                                  ),
                             'private' =>
                             array(
                                   'type' => 'Pluf_DB_Field_Integer',