Made two lines translatable

Addressing #10 and #11 in the OP of
https://github.com/HabitRPG/habitrpg/issues/4522

* "Send gift" (in the form that pops up when you click on username
label)
* "Invite Friends" button (in "Members" section in guilds and party)

common/locales/en/groups.json

@@ -101,5 +101,7 @@
     "messageAddedAsToDo": "Message copied as To-Do.",
     "messageWroteIn": "<%= user %> wrote in <%= group %>",
     "msgPreviewHeading": "Message Preview",
-    "leaderOnlyChallenges": "Only group leader can create challenges"
+    "leaderOnlyChallenges": "Only group leader can create challenges",
+    "sendGift": "Send Gift",
+    "inviteFriends": "Invite Friends"
 }

website/views/options/social/group.jade

@@ -60,7 +60,7 @@ a.pull-right.gem-wallet(ng-if='group.type!="party"', popover-trigger='mouseenter
         .panel-heading
           h3.panel-title
             =env.t('members')
-            button.pull-right.btn.btn-primary(ng-click="openInviteModal(group)") Invite Friends
+            button.pull-right.btn.btn-primary(ng-click="openInviteModal(group)")=env.t('inviteFriends')
         .panel-body.modal-fixed-height
           div.form-group(ng-if='::group.type=="party"')
             p=env.t('partyList')

website/views/shared/modals/members.jade

@@ -39,7 +39,7 @@ script(type='text/ng-template', id='modals/member.html')
         span.glyphicon.glyphicon-ban-circle
       button.btn.btn-md.btn-default(tooltip=env.t('sendPM'), ng-click="openModal('private-message',{controller:'MemberModalCtrl'})", tooltip-placement='right')
         span.glyphicon.glyphicon-envelope
-      button.btn.btn-md.btn-default(tooltip="Send Gift", ng-click="openModal('send-gift',{controller:'MemberModalCtrl'})", tooltip-placement='right')
+      button.btn.btn-md.btn-default(tooltip=env.t('sendGift'), ng-click="openModal('send-gift',{controller:'MemberModalCtrl'})", tooltip-placement='right')
         span.glyphicon.glyphicon-gift
     button.btn.btn-default(ng-click='$close()')=env.t('close')