Support variable distribution period length (#1691)

* Fix Vue error in groupProposalSettings

* Remove unused paymentTotalFromUserToUser getter

* Add groupSortedPeriodKeys getter in group.js

* Update getters to support variable period length

* Add isIsoString in time.js and use it

* Support variable period length in MonthOverview.vue

* Apply review

* Add periodStampsForDate in time.js

* Add getHistoricalPaymentPeriods in PaymentMixin.js

* Rename getSortedPeriodKeys to getAllSortedPeriodKeys

* Add a few comments

* Export new helpers from time.js

* Improve validation in periodStampsForDate

* Remove occurences of 'undefined'

* Fix missing break statement

* Fix wrong getter name in SupportHistory.vue

* fixup! Improve validation in periodStampsForDate

* Restore logging in Cypress

* Fix reactivity issue in MonthOverview.vue

* Fix data() in PaymentRowReceived/Sent.vue

* Use groupDistributionStarted i/o inWaitingPeriod

* Revert change in updateDistributionDate

* Use humanStart/DueDate in MonthOverview.vue

* Simplify PaymentDetail.vue/initializeDetails

* Use mounted i/o watch in PaymentRowReceived/Sent.vue

* Fix pedantic Flow error

* Pin contracts to 0.1.8

* Move mounted() near the top

* Revert humanDate to plain import in MonthOverview.vue

* Use payment.period to fix Invalid Date errors

* Add .start field in initPaymentPeriod

* Add .end field in in-memory payment periods

* Rename getPeriodPayment to getPaymentPeriod

* Fix issue 1739

* Pin contracts

* Add test for payment in 2nd period

contracts/0.1.8/chatroom.0.1.8.manifest.json

@@ -0,0 +1 @@
+{"head":{"manifestVersion":"1.0.0"},"body":"{\"version\":\"0.1.8\",\"contract\":{\"hash\":\"21XWnNSbTzDVk7QNPYi5n1C8w34ivhT2Xp91CWz1itVdhNjFqX\",\"file\":\"chatroom.js\"},\"authors\":[{\"cipher\":\"algo\",\"key\":\"<pubkey from deploy-key.json>\"},{\"cipher\":\"algo\",\"key\":\"<pubkey from alex.json>\"}],\"contractSlim\":{\"file\":\"chatroom-slim.js\",\"hash\":\"21XWnNN7LxitcRpsam5GmmsBJtwY7eekM5cBEdLUJNGCwPXnPE\"}}","signature":{"key":"<which of the 'authors' keys was used to sign 'body'>","signature":"<signature>"}}