multi: add basic paywall support

[jolan]

This adds basic support for requesting payments in DCR for users/proposals by adding some columns to the database for storing the payment address, amount, fulfillment tx, etc. for The www refclient has support for using the testnet faucet to pay fees.

The basic premise is that you'd setup a cold wallet and create two accounts
(politieauserpayments, politieaproposalpayments) and then
dcrctl getmasterpubkey on those accounts and plug those into
politeiawww and politeiad confs respectively and payments would go to addresses
belong to the accounts belonging to that cold wallet.

Closes #123.
Closes #125.

NOTE: This does nothing on the enforcement side yet. We still need to figure out some other
implementation details like revising the signup flow (#41) and whether or not we still want a
captcha (#29) and having a user landing page (#59).

I was thinking the 'create a new account' form should contain some
verbiage that explains that a fee of (user fee amount) payable in DCR is
required to create an account and an additional per-proposal fee of
(proposal fee amount) is also required for each proposal that way users
know exactly what they're signing up for. Then once they click
'Sign Up' they will go to a user landing page with a form that says:

Step 1) Please send x.xx DCR to TsExampleAddr
Step 2) Verify your email (disabled/greyed out)

There is a decent GUI for requesting payments and presenting a QR code
and polling and updating status in dcrpayments. It needs to be ported to
politeiagui/react though. (decred/politeiagui#106)

Since we wouldn't send the verification email until after payment is
confirmed, I don't think we'd need to worry about rate limiting signups. (#86)

I don't have any ideas for proposal enforcement since I haven't looked at that part
yet. It could be as simple as warning the admin that they shouldn't publish it because it's not paid.

[dajohi]

re-ok