[FIX] project: IE11+ issue with flex and overflow

IE11 and IE Edge presented issue in the project kanban view with content
overflowing its container.

On IE Edge it could be solved by "min-width:0" on the left part of a
kanban card, so it was an issue in the computation of widths order.

But on IE11 that wasn't enough so a less elegant fix has been used.

opw-653477

(solution found mainly by qsm)

addons/project/static/src/less/project_dashboard.less

@@ -8,6 +8,7 @@
     .o_project_kanban_main {
         .o-flex(1, 0, 0);
         padding: @odoo-horizontal-padding;
+        max-width: 70%; // for IE11 overflow issue
 
         // give space for manage section at the bottom
         margin-bottom: @odoo-horizontal-padding/2 + 19px;