Update NOTICE.txt (mattermost#2897)

- add new dependencies css-vars-ponyfill and react-overlays
- update copyright metadata on react-bootstrap

NOTICE.txt

@@ -213,6 +213,41 @@ THE SOFTWARE.
 
 ---
 
+## css-vars-ponyfill
+
+This product contains 'css-vars-ponyfill' by John Hildenbiddle.
+
+A ponyfill that provides client-side support for CSS custom properties (aka "CSS variables") in legacy and modern browsers
+
+* HOMEPAGE:
+ * https://jhildenbiddle.github.io/css-vars-ponyfill/
+
+* LICENSE: MIT
+
+MIT License
+
+Copyright (c) 2018 John Hildenbiddle
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+---
+
 ## emoji-regex
 
 This product contains 'emoji-regex' by Mathias Bynens.
@@ -1927,9 +1962,9 @@ SOFTWARE.
 
 ## react-bootstrap
 
-This product contains 'react-bootstrap' by Stephen J. Collings.
+This product contains a modified version of 'react-bootstrap' by Stephen J. Collings.
 
-Bootstrap 3 components built with React
+Bootstrap 4 components built with React
 
 * HOMEPAGE:
  * https://react-bootstrap.github.io/
@@ -2176,6 +2211,41 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 ---
 
+## react-overlays
+
+This product contains 'react-overlays' by Jason Quense.
+
+Utilities for creating robust overlay components
+
+* HOMEPAGE:
+ * https://github.com/react-bootstrap/react-overlays#readme
+
+* LICENSE: MIT
+
+The MIT License (MIT)
+
+Copyright (c) 2015 react-bootstrap
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+---
+
 ## react-redux
 
 This product contains 'react-redux' by Dan Abramov.