modules: Make luci-base sufficient to use luci apps

modules: Make luci-base sufficient to use luci apps

Per the discussion in https://github.com/openwrt/luci/issues/869, make
luci-base sufficient to login, logout, and review and apply or revert
uci changes.  This allows most luci-app-xxx to work without having
luci-mod-admin-full installed.

It has been tested with some apps and not luci-mod-admin-full, as well
as with luci-mod-admin-full (to make sure the usual case doesn't break).

Instead of creating a new module namespace (e.g. 'Base') we reduce the
opportunities for breakage by having luci-base take over the 'shell' of
the 'Administration' (admin/....) namespace.

Since admin is assumed by all current building LuCI components (including
Freifunk), this doesn't introduce the 'Administration' tab into any
situation where it would not already be present (but includes it where it
was before).

We also add a "Component not installed" page to avoid fatal errors and
backtrace when e.g. luci-mod-admin-full is not installed.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>