Subscribe to user

Problem
Based on some feedback (TODO: document feedback) subscribe or remote subscribe needs to be 'better'.

If you've had any issues with the current (pre 0.9.x) remote subscribe page, drop feedback here:
 * Find a better way to deal with "Can't add for another user!" case, when A is logged in and tries to subscribe to B, but enters in profile C

Purpose

 * Authenticate user's identity
 * Subscribe to user

UI
If the user is not logged in, the 'Subscribe' link on the Profile page should point to a page that outputs:

(Show entity_profile)

I have an account on this site:  |    |  

--- or -

I have an account elsewhere: Profile URL [   ] (e.g., http://identi.ca/nickname)

[Subscribe]

--- or -

I don't have an account, but, I want to create one quickly and subscribe: Nickname [   ] Password [   ] Password [   ]

[Create account and Subscribe]

Existing account
If a user has an account on the same site as the user that they wish to subscribe to, let them select from Login, OpenID or Facebook (iff the plugin is set up). Clicking on either one opens up their respective forms:

Nickname [   ] Password [   ]
 * Login

OpenID URL [   ]
 * OpenID

Connect with Facebook
 * Facebook

Login and OpenID options will have the following form submit button: [Login and Subscribe]

When they submit, log them in to the site, subscribe them to user and point them back to the user they were looking at.

Remote account
Base behaviour: If the visitor has an account on some other site, let them enter their profile URL. When they submit:
 * The remote site checks for credentials
 * Remote site asks the user go ahead or not with the process using Accept or Reject

If they Accept, point them back to the user they were looking at. In the page_notice area, tell them that they are now subscribed to them.

Optional: This widget specifically address the need for a user to remember/know their profile URL.
 * Whenever a user logs into their home site, they get cookied by a central site (e.g., widget.openmicroblogging.net).
 * Then other remote sites can use a JavaScript widget to either pre-fill the identity URL -OR- automatically submit. This skips straight from the publisher's profile page to the part where you approve the subscription on your home site.

New account
If the visitor needs an account, let them create one on the fly with the minimal requirements (e.g., Nickname, Password). On submit, create their account, subscribe them to the user and point them back to the user they were looking at.

Use page_notice to tell them that they created an account (welcome them!) and subscribed to that user successfully and suggest that they can update their account details. We could even point them to the Registration successful page.

Issues
Document issues with this proposal here