fix(views): refactor filter button slot in wrapper

Before this change, the filter button on the top right was positioned using absolute positioning and plenty of tricks, which were brittle and not really maintainable. Now, the buttons are positioned using flexbox, which should make this a lot more maintainable.

frontend/src/styles/components/_index.scss

@@ -1,5 +1,4 @@
 @import "tooltip";
 @import "labels";
-@import "project";
 @import "task";
 @import "tasks";

frontend/src/styles/components/project.scss

@@ -1,77 +0,0 @@
-// FIXME: should be a component <FilterContainer>
-// used in
-// - Kanban.vue
-// - Project.vue
-// - Table.vue
-
-$filter-container-top-default: -59px;
-$filter-container-top-link-share-gantt: -133px;
-$filter-container-top-link-share-list: -47px;
-
-.filter-container {
-  text-align: right;
-  width: 100%;
-  min-width: 400px;
-  max-width: 180px;
-  position: absolute;
-  right: 1.5rem;
-  margin-top: $filter-container-top-default;
-  z-index: 4;
-
-  display: flex;
-  justify-content: flex-end;
-
-  .button:not(:last-of-type) {
-    margin-right: .5rem;
-  }
-
-  .button {
-    height: $switch-view-height;
-  }
-
-  .card {
-    text-align: left;
-  }
-
-  @media screen and (max-width: $tablet) {
-    position: static;
-    margin: 0 0 1rem 0 !important;
-    max-width: 100%;
-    min-width: auto;
-
-    .items {
-      justify-content: center;
-    }
-
-    .search {
-      width: 100%;
-
-      .control:first-child {
-        width: 100%;
-      }
-    }
-  }
-}
-
-
-.link-share-container .gantt-chart-container .filter-container,
-.gantt-chart-container .filter-container {
-  right: 0;
-  margin-top: calc(#{$filter-container-top-link-share-gantt - 2} - 7rem);
-}
-
-.link-share-container .gantt-chart-container .filter-container {
-  margin-top: calc(#{$filter-container-top-link-share-gantt} - 5rem);
-}
-
-.link-share-container .list-view .filter-container {
-  margin-top: $filter-container-top-link-share-list - 10px;
-}
-
-.link-share-container.project\.table-view,
-.link-share-container.project\.list-view {
-  .filter-container {
-    right: 9rem;
-    margin-top: $filter-container-top-default;
-  }
-}