cleanup(test): Replace all homegrown email creation logic with `TestHelpers.createEmail()`

fixes #1608
chore(metrics): Change the event separator from `:` to `.`.

`:` is a special symbol for Kibana, using it here causes queries to be needelessly complex.

fixes #1601
Merge pull request #1627 from mozilla/issue-1608-create-email

cleanup(test): Replace all homegrown email creation logic with `TestHelpers.createEmail()`
chore(metrics): Round timestamps to the nearest hour instead of day.

* This allows us more granularity when querying the metrics DB, without sacraficing user privacy.

fixes #1602
Merge pull request #1628 from mozilla/issue-1601-metrics-event-separator

chore(metrics): Change the event separator from `:` to `.`.
fix(signin): cache credentials for desktop sign-ins, otherwise only cache email

Fixes #1621.
Merge pull request #1629 from mozilla/issue-1602-one-hour-metrics-timestamp

chore(metrics): Round timestamps to the nearest hour instead of day.
Merge pull request #1626 from mozilla/issue-1621-fully-logout

fix(signin): cache credentials for desktop sign-ins, otherwise only cache email

r=ckarlof,nchapman,vladikoff
Release v0.21.0
tests(oauth): fix cached credential tests for oauth sign in
Merge pull request #1633 from mozilla/fix-oauth-tests-cached-creds

tests(oauth): fix cached credential tests for oauth sign in
chore(docs): Update README instructions

[skip ci] Fixes #1631
Merge pull request #1634 from vladikoff/issue-1631-readme

chore(docs): Update README instructions
tests(signin): make cached credential tests more stable
Merge pull request #1636 from mozilla/improve-cached-creds-functional-tests

tests(signin): make cached credential tests more stable
Revert "tests(signin): make cached credential tests more stable"
Merge pull request #1638 from mozilla/revert-1636-improve-cached-creds-functional-tests

Revert "tests(signin): make cached credential tests more stable"
tests(signin): make cached credential tests more stable
Merge pull request #1641 from mozilla/improve-cached-creds-functional-tests-2

tests(signin): make cached credential tests more stable
tests(webdriver): Update Selenium version