add modal system, added/removed managed by vue, appearence managed by bootstrap

Modal component is an hybrid solution between :
- Vue3 modal implementation
  => with 'v-if:showModal' directive:parameter, html scope is added/removed not just shown/hidden
  => with slot we can pass content from parent component
  => some classes are passed from parent component
- Bootstrap 4.4 _modal.scss module
  => using bootstrap css classes, the modal have a responsive behaviour,
  => modal design can be configured using css classes (size, scroll)

See
* https://v3.vuejs.org/examples/modal.html#modal-component
* https://github.com/bootstrap-vue/bootstrap-vue/issues/5196

src/Bundle/ChillMainBundle/Resources/public/scss/chillmain.scss

@@ -66,34 +66,10 @@ div.vue-component {
    left: 0;
    width: 100%;
    height: 100%;
-   background-color: rgba(0, 0, 0, 0.5);
+   background-color: rgba(0, 0, 0, 0.6);
    display: table;
    transition: opacity 0.3s ease;
 }
-.modal-wrapper {
-   display: table-cell;
-   vertical-align: middle;
-}
-.modal-container {
-   width: 500px;
-   margin: 0px auto;
-   background-color: #fff;
-   border-radius: 0px;
-   box-shadow: 0 2px 8px rgba(0, 0, 0, 0.33);
-   transition: all 0.3s ease;
-   font-family: Helvetica, Arial, sans-serif;
-}
-.modal-header h2 {
-   margin-top: 0;
-   color: #42b983;
-}
-.modal-body {
-   margin: 0;
-}
-.modal-default-button {
-   display: block;
-   margin-top: 1rem;
-}
 
 /*
 * The following styles are auto-applied to elements with