OpenID: Document how to link multiple identities to an existing account

Linking another identity to an account is straightforward, but if done
wrong can result in a new account being created. This then requires the
administrator to manually fix up the database.

Update the documentation to describe the correct steps.

Also add a link to the wiki page describing how the administrator can
merge multiple accounts that have been accidentally created for the same
Gerrit user.

Change-Id: I9fe0664cdc00cb51c4523c1de63db2a83e38b480
OpenID: Mention Google OpenID shutdown in SSO documentation

Change-Id: I3803fc0a17c6c0077a8b9a3294bca02e816c19dc
OpenID: Remove support for Google Accounts

From May 19, 2014[1] Google is no longer allowing new servers to use
OpenID API to authenticate user accounts.  From April 20, 2015 Google
will shut down the OpenID service.

Delete Google Account suggestion from the web interface.

[1] https://developers.google.com/+/api/auth-migration#timetable

Change-Id: Idcf4e5a528e574c0042c897db87ea821fbf89315
Allow non lowercase hashtags.

There is no reason hashtags need to be lowercase. Using
lowercase acronyms or issue tags looks really strange.

Note: The search is still case insensitive, as it should be.

Change-Id: I8a1f733deef92a6d0ddc6b6292d3d0a657246aba
OpenID: Add support for Launchpad on the login form

The icon used in the login page is taken from [1] and encoded to
base64 using [2].

[1] https://launchpadlibrarian.net/16111289/gem-sm.png
[2] http://www.base64-image.de/

Change-Id: I8b3b10b7aef8a6dbae451edc017f9a35c1b9f1a9
Merge changes I8b3b10b7,Idcf4e5a5,I3803fc0a,I9fe0664c

* changes:
  OpenID: Add support for Launchpad on the login form
  OpenID: Remove support for Google Accounts
  OpenID: Mention Google OpenID shutdown in SSO documentation
  OpenID: Document how to link multiple identities to an existing account
Merge "Allow non lowercase hashtags."