luci-theme-bootstrap: simplify login dialog

luci-theme-bootstrap: simplify login dialog

Use an traditional HTML form with post submit action to the same URL in
order to simplify the login process and not rely on the discarded login
XHR reply properly setting the login cookie.

This will also avoid one useless request on login and hopefully fix login
issues reported with various browser environments.

As a bonus, the resulting code is somewhat smaller as well.

Ref: https://forum.openwrt.org/t/login-does-not-work/113360
Signed-off-by: Jo-Philipp Wich <[email protected]>
(cherry picked from commit c81b09ec3d83542b9d4be6297c45d0e66896265e)