Fixes the display of client counts, which are numbers and not strings
in the respondd data.

Fixes: 3a885a1b ("gluon-status-page: make "gateway nexthop" a link (#2278)")

Do not depend on the respondd-airtime module just to get the configured
channels. This removes the display of the frequency in addition to the
channel, as it is not readily available.

In addition, the translation string is improved to allow for text after
the channel number.

With this commit, the gateway nexthop is now a clickable link, that leads
to the status page of the nexthop.

This reverts commits
- caf2dd03.
- 07ebac6a
- 55eff45f

I accidentally pushed these commits as I had them lying around on a
dirty checkout I did testing on.

Let gluon-respondd expose "MemAvailable" from /proc/meminfo to allow for
a more realistic memory-usage estimation.

Information on MemAvailable can be found here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773

This new status page is significantly smaller than the old one. It always
loads its resources from the same host as the page itself, not requiring
cross-origin requests anymore.

It also uses the common i18n infrastructure of gluon-web.

Fixes #914