Source Selector: Refactor source selector

* Updates error message logic for when:
   * Source page does not exist
   * An equivalent target page exists
   * An equivalent target page exists and the chosen target title is in use
   * The chosen target title is in use

* Start translation button only enabled after entering a source title
  and choosing languages; disabled if source page does not exist

* New labels and message strings: From, To, placeholders

* Styling improvements to match design mocks

* Start translation on enter key if necessary fields are filled in

Change-Id: I6dbf7bfe79d5885e5507f3b3a22a488f8fbab6c0
Merge "Source Selector: Refactor source selector"