Add backend support for Hydra mount (#15482)

* chore: update time travelers shop to display seasonal backgrounds

* chore: update time travelers banner (note CSS borken rn)

* chore: fix borken CSS and update logic in shop

* chore: added isSubscribed function, not working

* chore: isSubscribed working but no bg for subscribers

* chore: logic and css updates

* chore: update habitica-images

* chore: add check for trinket

* chore: more time traveler shop logicking

* Add backend support for Hydra mount

- Add Dragon-Hydra to special mounts in stable.js
  - Configure as contributor level 7 reward with canFind: true
  - Add GIF format support for mount sprites
  - Enable admin panel granting capability

* Fix Vue template errors in timeTravelers component

* Fix duplicate template block in timeTravelers component

* add CSS for Hydra mount GIF sprites

Added CSS rules for Mount_Head_Dragon-Hydra and Mount_Body_Dragon-Hydra GIF sprites

* Remove the separate Hydra mount dimension declaration

---------

Co-authored-by: CuriousMagpie <eilatan@gmail.com>

website/client/src/assets/css/sprites.css

@@ -177,7 +177,7 @@
   height: 96px;
 }
 
-.Mount_Head_Gryphon-Gryphatrice, .Mount_Body_Gryphon-Gryphatrice {
+.Mount_Head_Gryphon-Gryphatrice, .Mount_Body_Gryphon-Gryphatrice, .Mount_Head_Dragon-Hydra, .Mount_Body_Dragon-Hydra {
   width: 135px;
   height: 135px;
 }
@@ -190,6 +190,14 @@
   background: url("https://habitica-assets.s3.amazonaws.com/mobileApp/images/BackerOnly-Mount-Body-Gryphatrice.gif") no-repeat;
 }
 
+.Mount_Head_Dragon-Hydra {
+  background: url("https://habitica-assets.s3.amazonaws.com/mobileApp/images/Mount_Head_Dragon-Hydra.gif") no-repeat;
+}
+
+.Mount_Body_Dragon-Hydra {
+  background: url("https://habitica-assets.s3.amazonaws.com/mobileApp/images/Mount_Body_Dragon-Hydra.gif") no-repeat;
+}
+
 .background_airship, .background_clocktower, .background_steamworks {
   width: 141px;
   height: 147px;