Configure quoted strings rule for yamllint

Add a configuration for the `quoted-strings` rule that matches our best
practices. The markdownlint configuration had to be updated to comply
with these new settings.

.mdl_config.yaml

@@ -6,12 +6,12 @@ default: true
 # MD003/heading-style/header-style - Heading style
 MD003:
   # Enforce the ATX-closed style of header
-  style: "atx_closed"
+  style: atx_closed
 
 # MD004/ul-style - Unordered list style
 MD004:
   # Enforce dashes for unordered lists
-  style: "dash"
+  style: dash
 
 # MD013/line-length - Line length
 MD013:
@@ -30,7 +30,7 @@ MD024:
 # MD029/ol-prefix - Ordered list item prefix
 MD029:
   # Enforce the `1.` style for ordered lists
-  style: "one"
+  style: one
 
 # MD033/no-inline-html - Inline HTML
 MD033:
@@ -42,19 +42,19 @@ MD033:
 # MD035/hr-style - Horizontal rule style
 MD035:
   # Enforce dashes for horizontal rules
-  style: "---"
+  style: ---
 
 # MD046/code-block-style - Code block style
 MD046:
   # Enforce the fenced style for code blocks
-  style: "fenced"
+  style: fenced
 
 # MD049/emphasis-style - Emphasis style should be consistent
 MD049:
   # Enforce asterisks as the style to use for emphasis
-  style: "asterisk"
+  style: asterisk
 
 # MD050/strong-style - Strong style should be consistent
 MD050:
   # Enforce asterisks as the style to use for strong
-  style: "asterisk"
+  style: asterisk

.yamllint

@@ -49,3 +49,14 @@ rules:
     forbid-explicit-octal: true
     # Do not allow implicit octal values (those beginning with a leading 0).
     forbid-implicit-octal: true
+
+  quoted-strings:
+    # Allow disallowed quotes (single quotes) for strings that contain allowed quotes
+    # (double quotes).
+    allow-quoted-quotes: true
+    # Apply these rules to keys in mappings as well
+    check-keys: true
+    # We prefer double quotes for strings when they are needed
+    quote-type: double
+    # Only require quotes when they are necessary for proper processing
+    required: only-when-needed