build: introduce official way to opt out of source minification

Introduce three new default enabled variables `LUCI_MINIFY_JS`,
`LUCI_MINIFY_LUA` and `LUCI_MINIFY_CSS`  that allow LuCI component
Makefiles to opt out of JavaScript, Lua and CSS compression respectively.

To e.g. inhibit CSSTidy postprocessing in a theme, the corresponding
Makefile can set `LUCI_MINIFY_CSS:=0` prior to including `luci.mk`.

Fixes: #5755
Signed-off-by: Jo-Philipp Wich <jo@mein.io>