* Rename Tag to Label.
* Improve UI to edit Project Label.
* Add scrollbar to view window and make edit window
auto-resizable(#156).
* Allow users to input category and name each separately(#157).
* Show License and Language even if they have no label(#155).
* Make the Edit button togglable.
* Add appropriate auto-focus on the input box.
* Allow to edit tags on Overview page.
* Remove the tag editor on Project Setting page.
* Make tag has its own category.
* Add defeault tags.
* Add a document for Tag Typeahead API.
Milestone has stored many calcaulted values (e.g. number of open issues)
persistently in database and it causes many problems. They should be
calcultaed just in time.
Also remove many lines of codes because this fix eliminates the needs.
Assignee is a user assigned to any issue in a particular project.
Advanced Search feature gets all assignees in a particular project
using this model and show them in assignee field in the search form.