mediawiki.action.edit: Move collapsibleFooter cookies to localStorage

mediawiki.action.edit: Move collapsibleFooter cookies to localStorage

The old cookies will become unused and expire automatically.

The new localStorage values won't have any expiry as localStorage
doesn't support it. We could provide ad-hoc expiry, but it wouldn't
be useful. It doesn't save space as it would only evict when visiting
the edit page (at which point the value is relevant again). Which means
it also doesn't help any privacy perspective (the value will be readable
for an undefined time until then, though never shared over the network).

If we want it to auto-expire, sessionStorage could be considered. Though
that means the value is reset everytime the user restarts their browser.

Bug: T110353
Change-Id: Ie9a4612de55e6aaf155a2179643e9b17f331e897