Option to load transactions after the rest of the page has loaded

Similar to DeferTranasctionLoading, but without the need for an extra
click.
Preserve page anchors when using delay loaded transactions

This jumps to the desired anchor in the page after the ticket history is
fully loaded and displayed.
Respect the full/brief header view when deferring and delaying history
Combine the two "TransactionLoading" configs, Defer and Delay, into one

The new config option is called ShowHistory and may be one of three
values: delay, click (previously defer), and always.  The default is
"delay" since it provides superior page load times.

A single option simplifies user preferences and makes it clear that this
is a "pick one" choice.  You can't have both delayed loading and click
to load.

An upgrade script takes care of porting DeferTransactionLoading
preferences forward.  For preferences which are explicitly false, we
remove the entry and set ShowHistory to "delay".  "always" may
technically be the most correct interpretation, but "delay" is the most
useful and still shows history without any interaction.
The new mode is called "delay", not "defer"
Merge branch '4.2/delay-transaction-loading'

Conflicts:
	docs/UPGRADING-4.2
	share/html/Ticket/Elements/ClickToShowHistory