fix(locale): add or reuse translation keys missing from all locales
Three translation keys used in the code did not exist in any locale,
so the raw key was displayed instead of a message:
- error.invalid_proxy_url, used when subscription discovery receives
an invalid proxy URL. It now uses error.invalid_feed_proxy_url,
which has the same text.
- page.tag_entry_count, used for the screen reader entry count on the
tag entries page. It now uses page.total_entry_count, like the feed
and category entries pages.
- error.unable_to_parse_html_document, used when feed discovery
cannot parse the HTML page. This key is added to all locales.