add missing td elements for table rows without help
new jelly tag <f:helpLink> for displaying help links

The new tag ensures that whenever there is no help available, a blank
table cell is output.
use <f:helpLink> in several tags
Merge pull request #1326 from albers/missing-td

fixed broken HTML tables, added Jelly tag for help links
Noting JENKINS-23872, JENKINS-24006, 19d8b80
Create a single point in which the basic authentication header is processed.

Previously, basic auth header was processed from two different servlet
filters in a single filter chain.

In case the 1st filter (ApiTokenFilter) manages to authenticate the
request, the 2nd filter (BasicProcessingFilter) tries to avoid
interpreting the API token as the password and failing authentication
(see BasicProcessingFilter.authenticationIsRequired), but the check
feels rather fragile.

Although I did eventually discover that the original problem (ZD-19640)
was not caused by this, I've already implemented & tested this change,
and this feels like a good work to be wasted, so I'm pushing this in
anyway.

Refrence: ZD-19640
divification of pane tag
<l:icon> tag