Account creation user experience/Strings

These are the system message changes involved in improving the login and create account forms. In the switch to the new forms bug 46333 in Gerrit #Iac7771 and subsequent changes, the following message keys changed.

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

Removed old messages

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

Changed but kept

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

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

(These messages are still used by preferences.)

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].