add tippy tooltips for mentions

yarn.lock

@@ -7311,6 +7311,10 @@ pngquant-bin@^5.0.0:
     execa "^0.10.0"
     logalot "^2.0.0"
 
+popper.js@^1.14.4:
+  version "1.14.4"
+  resolved "https://registry.yarnpkg.com/popper.js/-/popper.js-1.14.4.tgz#8eec1d8ff02a5a3a152dd43414a15c7b79fd69b6"
+
 portfinder@^1.0.13:
   version "1.0.17"
   resolved "https://registry.yarnpkg.com/portfinder/-/portfinder-1.0.17.tgz#a8a1691143e46c4735edefcf4fbcccedad26456a"
@@ -9425,6 +9429,12 @@ timsort@^0.3.0:
   version "0.3.0"
   resolved "https://registry.yarnpkg.com/timsort/-/timsort-0.3.0.tgz#405411a8e7e6339fe64db9a234de11dc31e02bd4"
 
+tippy.js@^3.0.3:
+  version "3.0.3"
+  resolved "https://registry.yarnpkg.com/tippy.js/-/tippy.js-3.0.3.tgz#ee543943808201e75d3a9369464576585d9d5305"
+  dependencies:
+    popper.js "^1.14.4"
+
 tmp@^0.0.33:
   version "0.0.33"
   resolved "https://registry.yarnpkg.com/tmp/-/tmp-0.0.33.tgz#6d34335889768d21b2bcda0aa277ced3b1bfadf9"