[FIX] runbot_merge: allow adding and removing release PR lines

In order to support partial freezing, we need the ability to remove
some of the release lines for the repos we don't want to
freeze (e.g. because they don't use per-version branches).

This subsequently means we need the ability to *create* new lines if
we fucked up and removed one we should not have. Alternatively the
freeze meat-bot could cancel the entire thing and redo the wizard but
that seems harsh and mean, so don't do that.

Fixes 0f3647b7c7 which specifically
mentioned partial freeze then proceeded to make them entirely
impossible anyway.

Part of #718

runbot_merge/models/project_freeze/views.xml

@@ -28,7 +28,7 @@
                         </p>
                         <field name="release_label" colspan="2"/>
                         <field name="release_pr_ids" nolabel="1" colspan="2">
-                            <tree editable="bottom" create="false" delete="false">
+                            <tree editable="bottom">
                                 <field name="repository_id" readonly="1"/>
                                 <field name="pr_id" options="{'no_create': True}"/>
                                 <field name="label"/>