feat(channels): an Added column, and About cards that wait for a resting pointer

The channel manager lists when each follow reached the list (the
subscription import that first saw it, or a subscribe made here) as a
sortable Added date after Last upload. The About cell's card opens after the
pointer rests on it for half a second instead of at once, and a scroll puts
an open one away.

frontend/src/i18n/locales/en/channels.json

@@ -82,6 +82,7 @@
     "stored": "Stored",
     "subs": "Subscribers",
     "lastUpload": "Last upload",
+    "added": "Added",
     "length": "Length",
     "tags": "Tags",
     "actions": "Actions"

frontend/src/i18n/locales/hu/channels.json

@@ -82,6 +82,7 @@
     "stored": "Tárolt",
     "subs": "Feliratkozók",
     "lastUpload": "Utolsó feltöltés",
+    "added": "Hozzáadva",
     "length": "Hossz",
     "tags": "Címkék",
     "actions": "Műveletek"