Refer to the nodejs-legacy package in INSTALL.md

Due to a name conflict, the main Node.js binary was renamed from
"node" to "nodejs" in Debian (https://bugs.debian.org/614907).
Packages to be installed via npm often don't expect this and fail to
install.

The long-term solution is to use system packages instead, though not
everything has been packaged yet; in particular, a Debian gulp package
is missing. So for now, recommend installing the nodejs-legacy package
which provides a symbolic link from "node" to "nodejs".
MBS-8091: When editing an AC, start from its actual content

The "Edit artist credit" edit applied clean_submitted_artist_credits
to the current content of the artist credit before comparing it to
the new artist credit (to determine if there was a change). This meant
that an existing artist credit that contained invalid characters or
non-normalized Unicode could not be fixed: Both sides were already
cleaned up, so the edit appeared to be a no-op and was ignored.

Use the actual current content of the artist credit as the old value
instead.
MBS-4668: Retain all query parameters on redirect

When given a page number that is too high, _load_paged redirects to
the last existing page. In the process, it used to store all query
parameters in a hash, thereby removing all duplicate keys. (In
particular, edit searches with multiple-value selects were affected.)
Use an array instead.
Improve page number validation
Merge remote-tracking branch 'chirlu/mbs-4668' into beta

* chirlu/mbs-4668:
  Improve page number validation
  MBS-4668: Retain all query parameters on redirect
Merge remote-tracking branch 'chirlu/mbs-8091' into beta

* chirlu/mbs-8091:
  MBS-8091: When editing an AC, start from its actual content
Merge remote-tracking branch 'chirlu/nodejs' into beta

* chirlu/nodejs:
  Refer to the nodejs-legacy package in INSTALL.md