Refine i18n

Changes:

* Add missing translation lookup
* Add missing keys in en locale file
* Remove unused translation keys (which were added into en file in localeapp.com)

But there are some more problems not fix yet. eg. some translation scope are inconsistent or duplicated:

* `views/forem/admin/forums/new.html.erb` is using `t("forem.forum.new")`. but `views/forem/admin/groups/new.html.erb` is using `t("forem.admin.group.new")`.
* `forem.admin.forum.new` and `forem.forum.new` are duplicate. Only `forem.forum.new` is in use.

If there is a plan to fix it?