add summary field to challenges and guilds (#8960)

* create new summary field for challenges

* finish implementating summary for challenges, add some support for guilds

* make small improvements to challenges code

* fix lint errors

* add more code to support summaries for guilds (still more needed)

* fix existing tests by adding summary field

* make existing tests pass

* WIP make "Public Challenges" text translatable

* change "leader" locale key to "guildOrPartyLeader" to make searches for it easier

* remove v-once from h2 headings

* remove failed attempt to localise text in <script>

* add quick-and-dirty error checking for guild not having categories

* make "Public Challenges" text translatable

* rename final ...PlaceHolder strings to ...Placeholder (lower-case "h") for consistency with existing Placeholder strings

website/client/components/challenges/challengeDetail.vue

@@ -56,7 +56,9 @@
       div(v-if='isLeader')
         button.btn.btn-danger(v-once, @click='closeChallenge()') {{$t('endChallenge')}}
     .description-section
-      h2(v-once) {{$t('challengeDescription')}}
+      h2 {{$t('challengeSummary')}}
+      p {{challenge.summary}}
+      h2 {{$t('challengeDescription')}}
       p {{challenge.description}}
 </template>