Make it easier to overwrite front end form help text regarding recurring

The help text on the front end pages is not always that appropriate. It may not actually be possible to cancel at any time and there is variation in sites about whether receipts are sent every donation or just for the first one.

There is a similar problem on the back-end forms. I logged a ticket for that (CRM-20250) but I'm going to propose a slightly different solution. On the back end form I think the sometimes-true text is of limited value & I think we could remove it.

On the front end form it seems more useful. I propose setting it from php as a variable, which would be easy to override in a hook. Ie. change this

<div id="recurHelp" class="description">

{ts}Your recurring contribution will be processed automatically.{/ts}
{if $is_recur_installments}
{ts}

You can specify the number of installments, or you can leave the number of installments blank if you want to make an open-ended commitment. In either case, you can choose to cancel at any time.

{/ts}
{/if}
{if $is_email_receipt}
{ts}You will receive an email receipt for each recurring contribution.{/ts} {/if}

</div>

to this

<div id="recurHelp" class="description">
{$recurringHelpText}
</div>

with the work done at the php layer.

This would fail fairly gently if something changed in the tpl, unlike a tpl override