Add a project-level configuration for PR builds

This allows users with the feature flag to enable/disable this feature.

docs/guides/autobuild-docs-for-pull-requests.rst

@@ -2,8 +2,11 @@ Autobuild Documentation for Pull Requests
 =========================================
 
 Read the Docs allows autobuilding documentation for pull/merge requests for GitHub or GitLab projects.
-This feature is currently available under a :doc:`Feature Flag </guides/feature-flags>`.
-So, you can enable this feature by sending us an `email <mailto:support@readthedocs.org>`__ including your project URL.
+This feature is currently enabled for a subset of our projects while being rolled out.
+You can check to see if your project has it enabled by looking at the :guilabel:`Admin > Advanced settings` and look for :guilabel:`Build pull requests for this project`.
+We are rolling this feature out based on the projects age on Read the Docs,
+so older projects will get it first.
+You can also ask for this feature by sending us an `email <mailto:support@readthedocs.org>`__ including your project URL.
 
 Features
 --------

readthedocs/projects/forms.py

@@ -203,6 +203,7 @@ class Meta:
             'analytics_code',
             'show_version_warning',
             'single_version',
+            'external_builds_enabled'
         )
         # These that can be set per-version using a config file.
         per_version_settings = (
@@ -259,6 +260,10 @@ def __init__(self, *args, **kwargs):
         else:
             self.fields['default_version'].widget.attrs['readonly'] = True
 
+        # Enable PR builder option on projects w/ feature flag
+        if not self.instance.has_feature(Feature.EXTERNAL_VERSION_BUILD):
+            self.fields.pop('external_builds_enabled')
+
     def clean_conf_py_file(self):
         filename = self.cleaned_data.get('conf_py_file', '').strip()
         if filename and 'conf.py' not in filename:

readthedocs/projects/migrations/0049_add_external_build_enabled.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.10 on 2020-05-18 20:17
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('projects', '0048_remove_version_privacy_field'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='project',
+            name='external_builds_enabled',
+            field=models.BooleanField(default=False, help_text='More information in <a href="https://docs.readthedocs.io/en/latest/guides/autobuild-docs-for-pull-requests.html">our docs</a>', verbose_name='Build pull requests for this project'),
+        ),
+    ]

readthedocs/projects/models.py

@@ -202,6 +202,12 @@ class Project(models.Model):
         ),
     )
 
+    external_builds_enabled = models.BooleanField(
+        _('Build pull requests for this project'),
+        default=False,
+        help_text=_('More information in <a href="https://docs.readthedocs.io/en/latest/guides/autobuild-docs-for-pull-requests.html">our docs</a>')  # noqa
+    )
+
     # Project features
     cdn_enabled = models.BooleanField(_('CDN Enabled'), default=False)
     analytics_code = models.CharField(