Send e-mail to users - specify format

Added ability to specify e-mail format (html/text) in sendmail function in Users Admin Api.

Then this is used in mailusers.tpl, where:
- selector is displayed for selecting format html or text. default is set to default format selected in Mailer module settings;
- notifydisplayhooks form_edit is placed in the form, .so Scribite editor to be able to be hooked;
- defaults are set for sender name and e-mail (from site settings).

| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #734
| License       | MIT