dev/core#2141 - "Add Mail Account" - Allow hookable listing of setup links

dev/core#2141 - "Add Mail Account" - Allow hookable listing of setup links

Overview
--------

For certain types of mail accounts -- such as Google Mail and Microsoft
Exchange Online -- the setup process may require interaction with a remote
web-service.  The web-service provides some critical details (like
authentication tokens) and some handy details (like their username / email
address).

If the supports one of these services, then this revision will enable a
setup process.  It alters the "Add Mail Account" action to use a more
fine-tuned procedure.

Before
------

* Navigate to "Administer => CiviMail => Mail Accounts".
* Below the table, there is a singular button "Add Mail Account".
* Click the button.
* It opens an empty form for configuring the account.

After
-----

By default, the UX is the same.  However, if you have an extension like
`oauth-client`, then it changes:

* Navigate to "Administer => CiviMail => Mail Accounts".
* Below the table, there is a select box for "Add Mail Account". It lists different account types.
* Choose one of the options from the dropbox. The step depends on...
    * If you choose "Standard Mail Account", it opens the empty config form.
    * If you choose "Microsoft Exchange Online", it redirects to MS to get authorization
      from the user. Then, it redirects and prefills the config form.