Add background project priority

Author: timbeccue

src/components/projects/CreateProjectForm.vue

@@ -155,6 +155,9 @@
               <option value="low_priority">
                 low priority
               </option>
+              <option value="background">
+                background
+              </option>
             </b-select>
           </b-field>
         </div>

src/store/modules/project_params.js

@@ -9,7 +9,7 @@ const state = {
   project_events: [],
   project_note: '',
   project_sites: ['common pool'], // can include 'common_pool' or any sitecode
-  project_priority: 'standard', // can be ['standard', 'time_critical', 'low_priority']
+  project_priority: 'standard', // can be ['standard', 'time_critical', 'low_priority', 'background']
   project_creator: {
     username: '',
     user_id: ''