luci-base: ui.js: fix multi vs. multiple keyword confusion

The "luci.form" and "luci.tools.widgets" classes use the "multiple"
keyword while ui.js uses "multi" internally, leading to single-value
dropdowns widget values getting stored as uci lists.

Fix the issue by using "multiple" everywhere.

Fixes: #2871
Signed-off-by: Jo-Philipp Wich <jo@mein.io>