i18n: translate page templates

[See how to translate a module](https://github.com/inveniosoftware/invenio-i18n/issues/78)

- [x] Page templates
  - [x] `page.html`
  - [x] `page_admin.html`
  - [x] `page_cover.html`
  - [x] `page_error.html`
  - [x] `page_settings.html`