Replace templates:tabs Atmosphere package with inline Blaze implementation

Drop the templates:tabs dependency as part of Meteor 3.0 migration prep.
The package is replaced by a lightweight inline basicTabs/tabContent
template pair with identical API, so no consumer template changes needed.

client/components/lib/basicTabs.jade

@@ -0,0 +1,11 @@
+template(name="basicTabs")
+  .basicTabs-container(class="{{name}}")
+    ul.tabs-list
+      each tabs
+        li.tab-item(class="{{isActiveTab slug}} {{class}}") {{name}}
+    .tabs-content-container
+      +Template.contentBlock
+
+template(name="tabContent")
+  section.tabs-content(class="{{isActiveTab slug}}" data-tab="{{slug}}")
+    +Template.contentBlock