Allows display_size to use a fixed unit for easy comparison of
multiple results, and fixed decimal places for the same reason.

Improves behaviour by using consistent decimal places and a
consistent space before the unit (the previous one only has a space
before 'bytes', not before 'KB').

Of existing uses, all the ones that displayed a 'maxbytes' type
configuration setting (which are likely to have an 'exact' size
and would be better shown as 512 KB rather than 512.0 KB) have been
changed to use 0 decimal places, to preserve previous behaviour.
All the uses which were showing an actual file or memory size have
been left as default (1 decimal place).

The patch allows for strings welcomeback and welcometosite to be
customised and use other user names than just the firstname.

Supported are placeholder properties matching all known name fields:
firstnamephonetic, lastnamephonetic, middlename, alternatename,
firstname and lastname. Special values fullname and alternativefullname
are supported, too.

By default, the firstname is kept for compatibility with the original
feature design.

* Plus use example domain for user email in the Behat test

A list of times for each cache key in a TTL cache is kept in a Redis
sorted list, which can be queried efficiently to delete expired
cache items later in a scheduled task.

This change makes set and delete 2x slower (only for caches which use
TTL) but there is no impact on get performance.

When using a try/catch for a find if the first match is not met then we
have an implicit 6 second delay waiting for the search to time out.

We can avoid this by combining the searches in a comma-separated
selection.

I have also taken the opportunity to convert the use of ->click() to the
i_click_on function which has the benefit of supporting wait for
pendingJS.

The standard NodeElement functions for getAttribute, getTagName,
getParent, and friends go back to WebDriver and parse the DOM for each
request. This is insanely slow per request, and in the case of forms we
do a lot of checking to determine the field type.

This change modifies the form field detection to copy the entire node
content into a DOMDocument and parse the document locally.

This is significantly faster - in some cases where there are large
documents minutes faster.

I believe that this should be a safe change as the document fetched from
the browser is normalised to match the doctype specified.