Replace Slack links for Mattermost links (#351)

Update links in `README.rst` and `docs`.

Author: santisoler

README.rst

@@ -30,8 +30,8 @@ discretize
 .. image:: https://img.shields.io/discourse/users?server=http%3A%2F%2Fsimpeg.discourse.group%2F
     :target: http://simpeg.discourse.group/
 
-.. image:: https://img.shields.io/badge/Slack-simpeg-4B0082.svg?logo=slack
-    :target: http://slack.simpeg.xyz
+.. image:: https://img.shields.io/badge/simpeg-purple?logo=mattermost&label=Mattermost
+    :target: https://mattermost.softwareunderground.org/simpeg
 
 .. image:: https://img.shields.io/badge/Youtube%20channel-GeoSci.xyz-FF0000.svg?logo=youtube
     :target: https://www.youtube.com/channel/UCBrC4M8_S4GXhyHht7FyQqw
@@ -122,4 +122,5 @@ Questions:
 http://simpeg.discourse.group/
 
 Chat:
-http://slack.simpeg.xyz/
+https://mattermost.softwareunderground.org/simpeg
+

docs/conf.py

@@ -246,7 +246,10 @@ def linkcode_resolve(domain, info):
     html_theme_options = {
         "external_links": [
             {"name": "SimPEG", "url": "https://simpeg.xyz"},
-            {"name": "Contact", "url": "http://slack.simpeg.xyz"},
+            {
+                "name": "Contact",
+                "url": "https://mattermost.softwareunderground.org/simpeg",
+            },
         ],
         "icon_links": [
             {
@@ -255,9 +258,9 @@ def linkcode_resolve(domain, info):
                 "icon": "fab fa-github",
             },
             {
-                "name": "Slack",
-                "url": "http://slack.simpeg.xyz/",
-                "icon": "fab fa-slack",
+                "name": "Mattermost",
+                "url": "https://mattermost.softwareunderground.org/simpeg",
+                "icon": "fas fa-comment",
             },
             {
                 "name": "Discourse",