refactor(client): No longer call fxa-js-client->signIn after signUp. signUp does everything it needs to do.

fixes #1840
feat(metrics): Report distinct metrics codes for missing and invalid emails

* Missing: error.<screen>.auth.1011
* Invalid: error.<screen>.auth.1023

fixes #1814
Use the newest fxa-js-client which contains the `unwrapBKey` functionality needed in signUp.
fix(client): Allow Fx18 to use the iframe flow.

Fx18 had two problems:
1) anchor.origin is not available and must be built from anchor.protocol and anchor.host.
2) anchor.protocol and anchor.host are only updated if the anchor's href is set via `anchor.setAttribute(href, ...`.

fixes #1969
cleanup(test): Use the `fillOutSignUp` functional helper to reduce duplication.
Merge pull request #1965 from mozilla/issue-1814-email-errors

feat(metrics): Report distinct metrics codes for missing and invalid emails

r=kparlante, zaach
Merge pull request #1970 from mozilla/issue-1969-iframe-fx18

fix(client): Allow Fx18 to use the iframe flow.
Merge pull request #1963 from mozilla/issue-1840-no-signin-after-signup

refactor(client): No longer call fxa-js-client->signIn after signUp. signUp does everything it needs to do.
refactor(client): Move interTabCommunication out of fxa-client into the views that need it.

interTabCommunication crept into fxa-client. The interTabCommunication functionality is only really necessary in complete_reset_password, so it has been moved there.

fixes #1807
Merge pull request #1966 from mozilla/issue-1807-inter-tab-channel

refactor(client): Move interTabCommunication out of fxa-client into the views that need it.

rebased, thanks @zaach!

r=zaach
fix(server): Allow the 500 and 503 pages to be iframed.

fixes #1978
Merge pull request #1979 from mozilla/issue-1978-iframe-error-pages

fix(server): Allow the 500 and 503 pages to be iframed.
Merge pull request #1971 from mozilla/cleanup-sign_up-functional-test

cleanup(test): Use the `fillOutSignUp` functional helper to reduce duplication.
feat(client): An Fx Sync relier can specify `customizeSync=true` to force the Customize Sync checkbox to be checked.

fixes #1954
Add the `importBooleanSearchParam` function to the search-param mixin.