React tables

This is a redo of #1502. I have done some architectural restructuring so that the base Table React component is more generalizable, and I've changed the way components get mounted onto the view (originally was using the react-rails gem, but now going a more native route). Also deleted unnecessary .rjs files (for the original tables) and partials, as well as the students' populate route in route.rb (we now populate by sending an AJAX request to the same url, just requesting it in JSON).
the list of column props is passed down from a higher-level component, which can specify whether a column should be sortable or not. For example, the actions column in the Groups tab of the assignment should not be sortable, so its columns look like: 
```javascript
var columns = [                                                                 
{                                                                               
id: 'name',                                                                   
content: '<%= j raw I18n.t('groups.groupe_name') %>',                         
sortable: true,                                                               
searchable: true                                                              
},                                                                              
{                                                                               
id: 'links',                                                                  
content: '<%= j raw I18n.t('actions') %>',                                    
sortable: false,                                                              
searchable: false                                                             
},                                                                              
{                                                                               
id: 'members',  
content: '<%= j raw I18n.t('members') %>',                                    
sortable: true,                                                               
searchable: true                                                              
}; 
```