Adds a new column/setting to groups, allow_unknown_sender_topic_replies, which is default false. When enabled, this scenario is allowed via IMAP:
* OP sends an email to the support email address which is synced to a group inbox via IMAP, creating a group topic
* Group user replies to the group topic
* An email notification is sent to the OP of the topic via GroupSMTPMailer
* The OP has several email accounts and the reply is sent to all of them, or they forward their reply to another email account
* The OP replies from a different email address than the OP (gloria@gmail.com instead of gloria@hey.com for example)
* The a new staged user is created, the new reply is accepted and added to the topic, and the staged user is added to the topic allowed users
Without allow_unknown_sender_topic_replies enabled the new reply creates an entirely new topic (because the email address it is sent from is not previously part of the topic email chain).
Groups page was loading fields that are only used on the group show
page, so move those fields to the GroupShowSerializer.
Also only fetch the default category and tag notifications once.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* Split alias levels in mentionable and messageable levels.
* Fixed some tests.
* Set messageable level to everyone by default.
* By defaults, groups are not mentionable or messageable.
* Made staff groups messageable by the system.
