Do not output spaces around subfields' titles

Having spaces inside the `<h3>` element creates artifacts in combination
with CSS rules like `h3:after { content: ...; }`.

BTW, for the sake of i18n, the whole sentence (text + colon) should be
generated, not just the field name.

PS: the patch can be backported without any changes to `stable/2.1.x` and `qa/2.1.x`.