amp-access: Perform logout via "login" config

[sebastianbenz]

Configure logout link via login config:

      "login": {
        "login": "<% host %>/amp-access/login/?rid=READER_ID&url=CANONICAL_URL",
        "logout": "<% host %>/amp-access/logout/?returnUrl=AMPDOC_URL"
      },

Then use for logout button

          <a on="tap:amp-access.logout" class="logout">Logout</a>

Explain advantage of having access to AMP runtime variables.

//cc @dvoytenko @andreban

[dvoytenko]

Small nit: it'd be tap:amp-access.login-logout. Doesn't sound great, but I guess we can always add it as a separate call in API at some point.