Create icon for Activity

To be included in the sidebar of the collapsible details (folderish and documents).
At the same level of the activity history, we will also have comments, so the chosen icon is meant to identify avitvity and general, with a 'comments thread feeling'.

Decided to keep the basic material design for consistency, but with a 90º rotation.

<i class="material-icons">
insert_chart
</i>