Account creation user experience/Strings

These are the message changes involved in improving the login and create account forms. In the switch to the new forms  in {gerrit|Iac777188002ca4cc1ea5043af244f217beab6eea}} and subsequent changes, the following message keys changed.

The earlier "ACUX" experiment involved other string changes, particularly from validation; for that, see /Experiment subpage.

SpecialUserlogin.php messages removed

 * userlogin -> either a specific 'createaccount' or 'login' in page heading and title
 * userloginnocreate -> gone. It was previously "Log in" (to distinguish from "userlogin", which was "Log in / create account").  This is no longer necessary since the login page heading and title is now always login-only.
 * gotaccount & gotaccountlink -> gone, we let the user login from the link in the top-right navigation
 * nologin & nologinlink -> gone, login form has a fancy userlogin-joinproject 'Join [wiki]' message.

Userlogin template messages removed

 * userlogin-resetlink -> userlogin-resetpassword-link (needs to be brief)
 * yourname -> userlogin-yourname without colon (in both forms)  (MessagesQqq said it's used in Preferences, but I don't see it)

Usercreate template messages removed

 * yourpasswordagain -> userlogin-yourpasswordagain without colon (MessagesQqq said it's used in Preferences, but I don't see it)
 * createaccountreason -> createacct-reason without colon

Userlogin template messages changes

 * remembermypassword -> userlogin-remembermypassword (and gone from createacct) BUT still in use in Special:ChangePassword
 * mailmypassword -> gone, users are always sent to a form to recover password BUT still in use in Special:ResetPassword
 * yourpassword -> userlogin-yourpassword (in both forms) BUT still in use in Preferences

Usercreate template message changes
(These messages are still used by preferences.)
 * createaccount -> createacct-submit button (and message that appeared in h2) BUT still in use in the skin
 * youremail -> createacct-emailrequired and createacct-emailoptional BUT still in use in Preferences
 * yourrealname -> createacct-realname BUT still in use in Preferences
 * new form eliminates the prefsectiontip div tip:
 * prefs-help-email-required is redundant with the placeholder text
 * Doesn't show the wordy prefs-help-email
 * Doesn't show the wordy prefs-help-email-others

TODO
TODO remove the colon from yourdomainname.

TODO? have we tested securelogin-stick-https

TODO Could/should rename showCreateOrLoginLink and createOrLoginHref, since the only link shown is login's [Join wiki].