QA: Refactoring browser tests

Simplified created_account_failure_messages and editor_ve features with
a more declarative style.

Refactored login steps and added a "I am using the mobile site" step for
explicit setup of mobile context.

Using API to set up Given context where appropriate. Using watch support
in the new API client.

Fixed some ArticlePage element selectors.

Implemented a ArticlePage#type_into method that helps generalize fill-in
behavior that works with both content-editable and textarea elements.

Tagged features with @vagrant that are known to work in
mediawiki-vagrant assuming all the necessary roles are enabled.

Change-Id: Icbd9958b42222343879a48f34147c9595e03bcdc
Merge "QA: Refactoring browser tests"