web/builder: add category filtering to builders page.

With this patch you can filter on category on the builders page.
example:
http://ci-master:8010/builders?category=maint

Change-Id: I3aba9ad9fce95e94b13971750403bec86dcc564b
Signed-off-by: Mattias Ryrlén <mattiasr@op5.com>
update comments for command_version

Based on information in #2969.
The `listdir` command requires slave version 2.16
only use obfuscated passwords on command_version 2.16 and higher

Fixes #2969.
check the slave version of a more sensible command
Merge pull request #1292 from djmitche/bug2969

Correctly handle obfuscation with older slaves

(a separate PR needs to be submitted for eight)


Fixes ticket:2969
backport GH-1223 to eight: Support specifying an identity file with GerritStatusPush
update the documentation

* the update is not exactly the same as in master
* a few things were added beside the actual backport
Merge pull request #1303 from sa2ajj/backport-gh-1223

Backport of GH-1223 to eight