doc: Add `build:` tag to list of prefixes.
doc: Add `translations:` tag to list of prefixes.
doc: rename `translations:` to `translation:`.
mod_base: Modal html title in z.dialog

Modal title was rendered as text element.
The problem was that htmlentities wasn't parsed at all:
i.e. &eacute; -> &eacute; instead of é
core: Keep file extension when it is allowed for the file's mime type

This fixes a bug where an .mp3 file was renamed to .m2a on upload,
because mimetypes returns multiple extensions for the mime type
audio/mpeg. Instead of taking the first extension found, it checks
whether the original filename has already an extension that is
allowed.
Update controller_static_pages documentation
Merge pull request #507 from yoonka/static-pages-update

doc: Update controller_static_pages documentation
mod_base: fixes a crash with is_a filter when input is rsc_alist and unknown cat