CSS: update rules for headings.

Specifies more information for <h2> and adds a rule
for <h3>.
CSS, Templ: add rule for "text" paragraphs.

This sets a maximum width of 80 ex to prevent them
from being unreadably wide.  Applied to the description
on the confirmation page.
Templ: move re-link lower down configuration page.

Also change heading levels to <h3> with a new <h2> at the top
for the configuration section.
Store: expose update_job method.

This acquires the lock and then calls the private
_update_job method.
Templ: add hidden data to confirm page.

The confirm page can now take a data dictionary containing
information which should be placed in hidden input elements.
Web: add ability to change Crab IDs.

There is now an extra form on the job configuration page
allowing the Crab ID to be changed.  This is necessary because
there is no other means provided for altering a Crab ID.
(Changing the Crab ID in a crontab and re-importing it
would be detected as a deletion and creation of a new job.)