If enabled, allow users and orgs to set their time machine expiration

Fixes https://www.pivotaltracker.com/story/show/142881203

data/model/user.py

@@ -24,6 +24,7 @@ from util.names import format_robot_username, parse_robot_username
 from util.validation import (validate_username, validate_email, validate_password,
                              INVALID_PASSWORD_MESSAGE)
 from util.backoff import exponential_backoff
+from util.timedeltastring import convert_to_timedelta
 
 
 logger = logging.getLogger(__name__)
@@ -78,7 +79,8 @@ def create_user_noverify(username, email, email_required=True, prompts=tuple()):
     logger.debug('Email and username are unique!')
 
   try:
-    new_user = User.create(username=username, email=email)
+    default_expr_s = _convert_to_s(config.app_config['DEFAULT_TAG_EXPIRATION'])
+    new_user = User.create(username=username, email=email, removed_tag_expiration_s=default_expr_s)
     for prompt in prompts:
       create_user_prompt(new_user, prompt)
 
@@ -188,7 +190,20 @@ def change_send_invoice_email(user, invoice_email):
   user.save()
 
 
+def _convert_to_s(timespan_string):
+  """ Returns the given timespan string (e.g. `2w` or `45s`) into seconds. """
+  return convert_to_timedelta(timespan_string).total_seconds()
+
+
 def change_user_tag_expiration(user, tag_expiration_s):
+  """ Changes the tag expiration on the given user/org. Note that the specified expiration must
+      be within the configured TAG_EXPIRATION_OPTIONS or this method will raise a
+      DataModelException.
+  """
+  allowed_options = [_convert_to_s(o) for o in config.app_config['TAG_EXPIRATION_OPTIONS']]
+  if tag_expiration_s not in allowed_options:
+    raise DataModelException('Invalid tag expiration option')
+
   user.removed_tag_expiration_s = tag_expiration_s
   user.save()