(Feature) Limit number of postcards sent per day

[fvictorio]

Closes #115.

This limits the number of postcards send in a given day to 10 (configurable). The counter is increased after the postcard was sent. When the limit is reached, no further registrations can be done.

[coveralls]

Pull Request Test Coverage Report for Build 378

• 36 of 57 (63.16%) changed or added relevant lines in 9 files are covered.
• 36 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-1.5%) to 72.716%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
web-dapp/src/components/RegisterAddressPage.js	3	4	75.0%
web-dapp/server-lib/postcard_limiter.js	12	14	85.71%
web-dapp/routes/prepare_reg_tx.js	5	7	71.43%
web-dapp/server-lib/session-stores/redis.js	6	8	75.0%
web-dapp/server-lib/session-stores/memory.js	2	5	40.0%
web-dapp/controllers/notifyRegTx.js	2	13	15.38%
<!--	Total:	36	57	63.16%	-->

Files with Coverage Reduction	New Missed Lines	%
web-dapp/server-lib/post_api.js	1	50.0%
web-dapp/server-lib/session-stores/memory.js	4	57.14%
web-dapp/server-lib/session-stores/redis.js	8	60.42%
web-dapp/src/components/MyAddressesPage.js	9	77.78%
web-dapp/src/components/ConfirmationPage.js	14	77.5%
<!--	Total:	36		-->

Totals
Change from base Build 370:	-1.5%
Covered Lines:	893
Relevant Lines:	1175

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💛 - Coveralls

[phahulin]

@fvictorio LGTM, except the error message that is displayed on the front-end when limit is reached. Right now it's general "Server error, bad request". I think this line can be changed https://github.com/poanetwork/poa-popa/blob/master/web-dapp/server-lib/send_response.js#L9 to always return 200, then error message will be displayed properly.

[fvictorio]

@phahulin Fixed. Returning a 200 wouldn't work because the promise chain needs to be interrupted, but I slightly modified the code that handles error messages in the front so that the error shows up.