This reworks the tabs CSS and javascript coding to be more flexible and to
support vertical tabs. (Bootstrap calls this stacked.) The javascript also
allows the URL to include the hash-tag of the last-clicked tab so that the
tab can be automatically shown when the page is reloaded.

This allows fields to specify a visibility constraint that will be evaluated
in real time in the browser to automatically show and hide fields that
should be hidden based on values of other fields in the same form.

Validation is not performed on fields server-side if they are considered
invisible when submitted.

Crowdin recreates YAML files with indent specifications which Spyc does not
currently handle:

body: |2
  body contents written
  here with 2 leading spaces

This is part of the Yaml spec after 1.0

backportme

Ticket numbers are now configurable as a system-default and by help topic.
Previously, an administrator could select between six-digit random and
sequential numbers. Now, the sequence is selectable so multiple sequences
as needed, and a format can be specified to define how the ticket numbers
look. Formats can also be defined for each help topic.

Remove retired navigation background image

Fixes #145

And use admin-defined site logo