WordCamp Docs: Add a currency field for the sponsorship agreement

[coreymckrill]

The sponsorship agreement template has USD hard-coded as the currency type when it generates a PDF to send to a prospective WordCamp sponsor. However, sponsors are allowed to pay their sponsorship in the local currency where the WordCamp is located, and this is often the easiest choice for them.

This enhancement needs two components:

• An additional field to specify the currency type
• Including and sanitizing the currency type in the template file used to generate the PDF

[tyxla]

Do not hardcode currency in the sponsorship agreement template. Add a hint for adding the currency to the amount fields.

[tyxla]

I've attached a patch for this one. It suggests the following approach (as discussed on Slack in #meta-wordcamp):

• Don't hardcode the currency (US $) in the sponsorship agreement template.
• Add a hint to both amount fields that currency name (and currency symbol) should be added to the amounts.

This approach seems easier to me, because this way we don't need to care about the position of the currency symbol (whether it's before or after the amount).

[coreymckrill]

In 5607:

WordCamp Docs: Allow non-USD currencies in sponsorship agreement

This simply removes the hard-coded "$US" from the template and adds
notes to the generation form asking the user to include the currency
type when they enter values. This makes it possible to generate a
sponsorship agreement that specifies a local currency.

Fixes #2914
Props tyxla