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