Add a project feature flag for showing invoices in EUR - #6644

If the feature flag is set, convert the invoice from USD to EUR at the
given eur_rate. When this is done, use the EU bank transfer info,
except avoid the Intermediary BIC, because that should only be needed
for USD transfers (I think).

While working on this, I found that we were inconsistent in regards
to rounding:

* Invoice#charge: Use rounded value
* project billing_info pay route: Use floored value (.to_i)

I also found that the implicit rounding when showing the total value
could potentially not match the amount charged. This is because of
a difference in how Ruby rounds:

* `1.005.round(2) # => 1.01`
* `"%0.02f" % 1.005 # => "1.00"`

Both of these seem like bad ideas. It's only a single cent, but the
optics of charging a different amount than is displayed (especially
when the displayed amount is lower) are quite bad.  I decided to be
consistent and make the project billing_info pay round use the rounded
value as well.

Rounding the amount up at any point feels customer unfriendly. We
should consider always rounding down, but this only attempts to
make the behavior consistent, it doesn't make that change.

This displays the conversion rate using 4 decimal places, since from
my research, that is standard pip.

This marks plain text emails with the UTF-8 charset, now that they
can potentially include the euro sign.