Implement feature 'Viewing Tops'
Move model associations back from contexts to ActiveRecord classes
Add gem 'erector'
Create a TopsByTagFieldPresenter based on Erector

* Uses this new presenter in GlobalViewPresenter in order to render
	the tops
* Deletes context 'ViewingTopTaggings' (temporary, as we'll need it later
  to implement access control on taggings, and event logging)
Refactor PersonPresenter to use Erector::Widget
Refactor ProjectPresenter to use Erector::Widget
Refactor PeoplePresenter to use Erector::Widget
Remove context 'HavingAGlobalView'
Refactor GlobalViewPresenter using Erector::Widget, remove HomePresenter
Refactor ProjectsPresenter using Erector::Widget
Refactor TagsPresenter using Erector::Widget
Refactor TagPresenter using Erector::Widget
Refactor TaggingsPresenter using Erector::Widget
Remove unused files
Cleanup presenters
Rename TaggingsPresenter -> TagFieldWithTaggingsPresenter
Extract ViewingATagTaggings html-building code to TagTaggingsPresenter
Adjust whitespace around table captions
Remove unused devise view files