This lets us implement changing signatures and deleting bottom-quoted
messages without having to reload the whole view, and makes it possible
to target only the user's content when modifying for send, etc.

* src/client/composer/composer-web-view.vala (ComposerWebView): Move
  composer CSS into composer-web-view.css resource file, load it when
  loading JS resource and add it to the view's user content manager.
  (ComposerWebView::load_html): Split up body, signature and quote into a
  DIV container for each.
  (ComposerWebView::linkify_content): Replaced with ::clean_content,
  which will also tidy up internal markup before sending. Update call
  site and unit test.

* src/engine/rfc822/rfc822-utils.vala (Geary.RFC822.Utils): Remove some
  more obtrusive white space when sending replies/forwards.

* test/client/composer/composer-web-view-test.vala,
  test/js/composer-page-state-test.vala: Update tests to expect new HTML
  and text output from ComposerWebView and use of individual parts for
  composer markup.

* ui/composer-web-view.js (ComposerPageState): Replace messageBody
  property and uses with bodyPart, signaturePart and quotePart. Set these
  content-editable on load. Move listeners from messageBody back to the
  document.body so they also listen for events on the additional
  parts. Keep track of text cursor location within the parts and set a
  class if so, to work around the lack of :focus-inside support.
  (ComposerPageState::updateSignature): Implement by updating the inner
  content of the signature part.
  (ComposerPageState::deleteQuotedMessage): Implement by removing the
  quote part from the DOM tree.
  (ComposerPageState::containsAttachmentKeyword): Consider only the
  bodyPart when scanning for attachments, remove hacks for ignoring the
  signature any any quoted message.
  (ComposerPageState::linkifyContent): Mirror ClientWebView change and
  replace with ::cleanContent. Ensure existing parts have contenteditable
  and focus class removed, remove signature and quote parts if empty.
  (ComposerPageState::getHtml): Generate HTML using clones of the three
  parts, so we can rmeove contenteditable and focus classes without
  modifying the actual DOM.
  (ComposerPageState::selectionChanged): Update focus class on parts as
  needed.