🧹 Tags: remove double get, sort tags (#5620)

Two tiny changes ported over from #5431:

- Remove a duplicate database retrieval when looking up the tags of an adventure.
- Sort the tags (when they are saved to the database).

**How to test**

Unsorted tags should be sorted after adding a tag. Otherwise no functional difference.

tests/cypress/e2e/for-teacher_page/customize-adventure_page/tags.cy.js

@@ -66,10 +66,10 @@ describe("Tags of adventures", () => {
     }).as("deleteTag")
 
     cy.wait(500)
-    cy.get("#tag_2")
+    cy.get("#tag_1")
       .should("be.visible")
       .should("include.text", "statements")
-    cy.get("#tag_2 .fa-circle-xmark")
+    cy.get("#tag_1 .fa-circle-xmark")
       .click()
     cy.wait("@deleteTag").should('have.nested.property', 'response.statusCode', 200)
     cy.get("#tags_list li")

website/tags.py

@@ -30,7 +30,6 @@ def get_public_tags(self, user, adventure_id):
         adventure_id = request.args.get("adventure_id")
         adventure = self.db.get_adventure(adventure_id)
         if adventure:
-            adventure = self.db.get_adventure(adventure_id)
             # exclude current adventure's tags
             public_tags = list(filter(lambda t: t["name"] not in adventure.get("tags", []), public_tags))
 
@@ -58,6 +57,7 @@ def create(self, user, adventure_id, new_tag=None):
         adventure_tags = db_adventure.get("tags", [])
         if tag_name not in adventure_tags:
             adventure_tags.append(tag_name)
+            adventure_tags = sorted(adventure_tags, key=lambda tag: tag)
             self.db.update_adventure(adventure_id, {"tags": adventure_tags})
         else:
             return make_response(gettext("tag_in_adventure"), 400)