webview: Escape HTML in displayed errors.

In addition to the code in src/webview/html/, there's this one bit in
js.js where we build up some HTML.  Escape the data we interpolate
here too.

Because this code runs inside the webview and can't import from
our main codebase, and there's only a little of it, skip the fancy
template tag in favor of manually invoking an escape function.
On the other hand, take advantage of the browser environment to
implement that escape function very simply.

ui: Improve compose box buttons and spacing

* more consisntency in padding of compose input between platforms
* larger touch areas for all buttons
* move all related styles to composeBoxStyles
* buttons on left and right are precisely vertically aligned

webview: Queue unsent events until webview ready to process

* waits for 'ready' event and tracks status in `isReady`
* if it needs to send an event but not yet ready, queue in `unsentMessages`
* when ready send if any messages were queued

Add support to getNarrowFromNotificationData for group narrows

For this to work we need to parse a string of comma delimited
numbers of the recipients. This data is not yet passed to the
function; we keep the function flexible and functional by making
the parameter of `usersById` optional.`

webview: Pass baseUrl and authorization headers

* ensures that relative URLs are resolved correctly
* allows loading of any resource needing authorization

passwordAuthScreen: Fix two taps being required to hit "Log in".

One was used to remove focus from input & dismiss keyboard.
And second one was captured by the button.
To fix this pass keyboardShouldPersistTaps="always" to the
scrollView.

https://facebook.github.io/react-native/docs/scrollview.html#keyboardshouldpersisttaps

fix: Final fix for iOS notifications

Read data from a key "zulip"

refactor: Make notification handling more consistent

* no need for async for handlePendingNotifications
* proper name for the event handler
* handlePendingNotifications() in iOS the same as Android

flow: Add types for the config object

Fixes Flow complaining about config.startup.notification

Fix Stream title alignment issue on Android