Change indent settings in .editorconfig

Generally we seem to use 2-space indents, e.g. in the CSS of Encoding, or the JavaScript of Streams, or the .travis.yml files everywhere. It's only in the Bikeshed file that we use one-space indents. Additionally, per https://www.python.org/dev/peps/pep-0008/#indentation, Python files use four-space indents.

This updates .editorconfig appropriately, and aligns .pr-preview.json files with this new default.

.editorconfig.template

@@ -4,13 +4,16 @@ root = true
 end_of_line = lf
 insert_final_newline = true
 charset = utf-8
-indent_size = 1
+indent_size = 2
 indent_style = space
 trim_trailing_whitespace = true
 max_line_length = 100
 
 [Makefile]
 indent_style = tab
 
-[.travis.yml]
-indent_size = 2
+[*.bs]
+indent_size = 1
+
+[*.py]
+indent_size = 4

.pr-preview.json.template

@@ -1,9 +1,9 @@
 {
-    "src_file": "@@bs@@.bs",
-    "type": "bikeshed",
-    "params": {
-        "force": 1,
-        "md-status": "LS-PR",
-        "md-Text-Macro": "PR-NUMBER {{ pull_request.number }}"
-    }
+  "src_file": "@@bs@@.bs",
+  "type": "bikeshed",
+  "params": {
+    "force": 1,
+    "md-status": "LS-PR",
+    "md-Text-Macro": "PR-NUMBER {{ pull_request.number }}"
+  }
 }