Fix Cache-Control header of non-versioned assets

Non-cache-busted assets such as /dist/core-main.js also matched the
regex meant for cache-busted assets (note the ? at the end of the
regex).
The FilesMatch directive for cache-busted assets coming after the
non-cache-busted version all assets actually got the immutable flag
in their Cache-Control header. This caused client-side errors on
updates.

Query strings are not actually passed to FilesMatch directives so we
need another way to tell cache-busted/versionned assets apart from
non-versioned assets, here using If/Else directives.

Signed-off-by: Nicolas Guichard <nicolas@guichard.eu>