Move style for user search dropdown to scoped style

In 60c2bad, the user search dropdown was widened to fill the width of the modal - this was achieved using !important in the Primevue stylesheet.
This has been moved to a locally scoped style inside SearchUsers.vue instead, eliminating the need for !important.

frontend/src/assets/primevue.scss

@@ -74,10 +74,6 @@
   }
 }
 
-.p-dropdown.usersearch {
-  width: 100% !important
-}
-
 .p-checkbox,
 .p-radiobutton {
   &.p-checkbox-checked,

frontend/src/components/form/SearchUsers.vue

@@ -152,7 +152,7 @@ onMounted(() => {
           :option-label="(option) => getUserDropdownLabel(option)"
           editable
           :placeholder="userSearchPlaceholder"
-          class="mt-1 mb-4 usersearch"
+          class="mt-1 mb-4"
           :class="invalidSelectedUser ? 'p-invalid' : ''"
           @input="onInput"
           @change="onChange"
@@ -177,6 +177,6 @@ onMounted(() => {
 
 <style lang="scss" scoped>
 .p-dropdown {
-  width: 60%;
+  width: 100%;
 }
 </style>