PLAT-75 migrate HAWelcome job to a task

This pull request moves the `HAWelcome` job into a task. As part of this process I'm also:

 * Adding a `.setup.php` file with the extension setup.
 * Moving the hooks into `HAWelcomeHooks`
 * Moving the task logic into `HAWelcomeTask`
 * Moving the job run logic into `HAWelcomeTask::sendWelcomeMessage`
 * I'm also simplifying the `sendWelcomeMessage` logic and adding unit tests
 * Adding a dispatcher class for the hook logic that can be tested

/cc @michalroszka @Wikia/platform-team 
Wouldn't it be better to check ```$wgCommandLineMode``` first, instantiate ```$dispatcher```, and then check ```$dispatcher-> welcomeMessageDisabled()```? This if check looks the same.