Allow extensions to use custom topic icons

Currently extensions are unable to use custom topic icons because the icons path is hardcoded in T_ICONS_PATH. Thus, extension would have to copy its custom icons into standard phpBB folder which would be undesireble extension behavior.

The idea is to get rid of T_ICONS_PATH everywhere by mean of storing the full relative icon path in ICONS_TABLE/icons_url (for example: ext/ext_vendor/ext_name/icons/icon.png). Build-in icons path config value should be keeped as is to operate icons management in regular way when they are added via ACP as usually.