(Bug 177) finished basic /admin/sendmail page

The requested scope of this bug is in three parts: send message, lookup sent
 messages, and manage form messages.  This is only the first of those three
 parts.

This is a convenience form for sending emails to users from registered site
 addresses such as support@dreamwidth.org.  Each "from" address is tied to a
 particular privilege using a new hash in config-local.pl named
 %LJ::SENDMAIL_ACCOUNTS.  The keys of the hash are the email account names
 and the values of the hash are the priv:arg pairs needed to send a message
 with that account.

The original bug spec said to use userlog to record when an email is sent.
 I did that, but because userlog transactions are necessarily keyed to the
 user who took the action and limited in how much additional information they
 can contain, I also added a new global table, siteadmin_email_history, for
 recording the details of each message sent.  Due to the pressure of time,
 the interface for looking up sent emails hasn't been added yet, but the
 information is being added to the database for later querying.
(Bug 177) only give access to users who have siteadmin:sendmail priv
Merge pull request #803 from kareila/release-bug0177-sendmail

(Bug 177) finished basic /admin/sendmail page