Set content.cookies.accept to all by default

Setting it to no-3rdparty is a nice idea from a "privacy by default"
perspective, but it also breaks various pages.

In particular, due to what's probably a Qt bug, it breaks the new GMail:
https://bugreports.qt.io/browse/QTBUG-71393

Let's be in line with what other browsers by default here.

Fixes #4281

Author: The-Compiler

doc/help/settings.asciidoc

@@ -1591,11 +1591,11 @@ Type: <<types,String>>
 Valid values:
 
  * +all+: Accept all cookies.
- * +no-3rdparty+: Accept cookies from the same origin only.
+ * +no-3rdparty+: Accept cookies from the same origin only. This is known to break some sites, such as GMail.
  * +no-unknown-3rdparty+: Accept cookies from the same origin only, unless a cookie is already set for the domain. On QtWebEngine, this is the same as no-3rdparty.
  * +never+: Don't accept cookies at all.
 
-Default: +pass:[no-3rdparty]+
+Default: +pass:[all]+
 
 On QtWebEngine, this setting requires Qt 5.11 or newer.
 

qutebrowser/config/configdata.yml

@@ -329,15 +329,16 @@ content.cache.appcache:
     ask the application cache for the contents, before hitting the network.
 
 content.cookies.accept:
-  default: no-3rdparty
+  default: all
   backend:
     QtWebKit: true
     QtWebEngine: Qt 5.11
   type:
     name: String
     valid_values:
       - all: "Accept all cookies."
-      - no-3rdparty: "Accept cookies from the same origin only."
+      - no-3rdparty: "Accept cookies from the same origin only. This is known
+          to break some sites, such as GMail."
       - no-unknown-3rdparty: "Accept cookies from the same origin only, unless
           a cookie is already set for the domain. On QtWebEngine, this is the
           same as no-3rdparty."