Clean up template locale usage (#27856)

After many refactoring PRs for the "locale" and "template context
function", now the ".locale" is not needed for web templates any more.

This PR does a clean up for:

1. Remove `ctx.Data["locale"]` for web context.
2. Use `ctx.Locale` in `500.tmpl`, for consistency.
3. Add a test check for `500 page` locale usage.
4. Remove the `Str2html` and `DotEscape` from mail template context
data, they are copy&paste errors introduced by #19169 and #16200 . These
functions are template functions (provided by the common renderer), but
not template data variables.
5. Make email `SendAsync` function mockable (I was planning to add more
tests but it would make this PR much too complex, so the tests could be
done in another PR)

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