luci-base: fix field section add button/input field names

Some CBI map models, mainly the Network -> VLAN page, expect a valid
previous section ID in their Section:create() callback.

Previous refactoring of the tblsection markup broke this behaviour as
the "section" loop variable was accidentally localized, causing it to
be undefined outside of the loop body which caused the section add
button and name input fields to get rendered with a wrong "name"
attribute.

Fix this by moving the "section" variable declaration out of the loop
and by readding references to it in the non-anonymous section add case.

Fixes FS#1657
Fixes 002c4d1d

 ("luci-base: add "Name" label to autogenerated title column")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>